If a fighter is wearing Spiked Gauntlets wielding a Ranseur, does he threaten both the 5' area around him with the piked gauntlets and the 10' with the Ranseur which would allow him to make an attack of opportunity with either weapon depending on the distance of his opponent?
Letting go of your polearm and punching
This was my immediate assumption of how you imagined this going, and it might work but it’s on very thinnest of ice, officially.
First, you need to have an answer to a previous question: Can you take free actions during attacks of opportunity?
If you cannot, then you definitely cannot let go of the polearm to take an attack of opportunity: it’s a free action to let go of the polearm.
Even if you can, it’s unclear if the fact that you could let go of the polearm to hit someone with the spiked gauntlet means you threaten that space. One could easily say that, even though you could let go of it once you are provoked, before you are provoked you aren’t performing an action and therefore cannot let go of the polearm and therefore do not threaten anything.
You can also make a balance-based argument, saying that not threatening interior squares is an intentional drawback of reach weapons that should not be so easily thwarted. Spiked chains (which threaten near and far) are special for a reason. And there are concerns about other free actions you might take during an attack of opportunity and the problems those might cause.
Ultimately, getting free actions during attacks of opportunity is not explicitly spelled out in the rules, and requires fairly “optimistic” reading between the lines. It requires assuming an exception exists even though none is explicitly listed—somewhat dubious. And extending that to allowing you to threaten squares based on a free action you might take is possibly more dubious still. And the limitation on reach weapons not threatening near you is definitely real and intended.
But I still allow it. I think the “implicit” exception makes vastly more sense, and that harping on its absence is fairly pedantic. I think that the fact that a polearm and spiked gauntlet requires investing in two separate weapons still gives spiked chains a large advantage. And plenty of other options—armor spikes, unarmed strikes—don’t require letting go of anything and absolutely work. No free action during an attack of opportunity has ever been a problem for my games. Finally, frankly, I think attacks of opportunity are one of the only nice things that martial characters get, so I think it’s good to allow them to actually do so. So I acknowledge that you probably can’t, strictly speaking, but I think the game is better off if you can—and that there’s enough room there to allow it.
Punching while holding your polearm
This didn’t even occur to me as an option at first. I always assumed your hand had to be free in order to attack with a gauntlet, spiked gauntlet, or unarmed punch, but I cannot find that anywhere in the rules.
Certainly, from a narrative or simulationist standpoint, it’s not hard to imagine the maneuver, though some weapons would probably interfere (punching daggers spring to mind). But a polearm? Easy to imagine.
And from a gamist perspective, since non-punch unarmed strikes, or armor spikes, are also options, it doesn’t really add anything to allow punching, whether with a bare hand or a spiked gauntlet.
The only real argument against it that I can find is that unarmed strikes are specifically unarmed. Arguably, if you are armed with some weapon, you cannot use one. Except we know that isn’t true, since you can still kick. And spiked gauntlets aren’t unarmed anyway, despite the similarity (and unlike regular gauntlets, which reference unarmed strikes, spiked gauntlets are written as entirely separate).
So yeah, given the lack of a rule saying you can’t, and the lack of compelling narrative, simulationist, or gamist reasons to prevent you, it certainly seems to me like you could.
I would allow both letting go of the polearm to punch, and punching with the polearm in hand. I would allow both cases to happen without requiring extra investment or enforcing additional penalties. Either maneuver is equivalent to attacking with armor spikes or a (non-punch) unarmed strike anyway, so there is no particular reason why this approach should be worse, to my mind.
I might object to punching with certain weapons in hand. For instance, a punching dagger stabs someone when you punch them: it’s hard to imagine an effective way to punch someone without stabbing them while you have a punch dagger on hand. Maybe an overhand smash, hitting with the bottom of your fist while the blade sticks out to the side, but that might be awkward. “Might be awkward” sounds like a reasonable place to put in a small attack penalty; probably just −1 though, since you can see how it would be done and it doesn’t really take fantastic skills to make it happen.
But punching with a polearm, or punching with a polearm in the other hand, doesn’t really seem awkward at all. Those I would just let happen.
Can a Spiked Gauntlet worn by a polearm-wielding Fighter allow him to do attacks of opportunity with it?
At least one designer assumes, YES
According to the Rules Compendium, pp 5
For example, the rules don’t come out and say that a Medium creature threatens all squares within 10 feet while wielding a reach weapon and wearing spiked gauntlets. However, it’s appropriate to assume the creature does just that.
However, that's from the introduction, under Rules Basics->Adjudication, under a discussion on how
No set of rules can cover every possible circumstance
And it's qualified as an assumption that's obvious to the designer and author but clearly not to everyone or this question would not have been asked.
Without any other rules source, I would suggest, It's up to your DM
Let us assume polarm means 2-handed weapon with reach. There are other polearms but they aren't germane.
There are no explicit rules allowing or denying this.
However, if you read the requirement from threatening,
You threaten all squares into which you can make a melee attack
And apply it to the weapon your wielding because
If you’re unarmed, you don’t normally threaten
And extrapolate that threatening is intrinsically tied to the weapon your wielding, then you have to first ask your DM, can I wield more than one weapon with the same hand (or appendage)?
While a spiked gauntlet is a manufactured weapon, and it is implied it can be worn while weilding another (manufactured) weapon, whether or not that means "wielded" as in able to make a melee attack is unclear.
I'm going to suggest No for a few reasons.
While not explicitly stated anywhere, the concept of limiting a hand or appendage to wielding one weapon is littered throughout two-weapon fighting, natural weapons and the equipment entries.
The rules for natural weapons and two-weapon fighting only ever mention wielding one as your primary and one as your off hand or either a manufactured weapon or a natural weapon with a given appendage, but not both. Specifically, the full attack entries for monsters never list the same appendage using more than one weapon (natural or manufactured). Marilith, for example.
This question is about Attacks of Opportunity, not two-weapon fighting or natural weapons, but if you could wield more than one weapon with an appendage, it would show up in those entries.
This concept also follows from this small tidbit from Armor Spikes;
You can’t also make an attack with armor spikes if you have already made an attack with another off-hand weapon
Given that, if your DM decides you cannot wield two weapons at the same time, you have to choose either to threaten adjacent (wielding spiked gauntlets while holding a polearm) or with reach (wearing spiked gauntlets while wielding a polearm). You also cannot switch at the time your enemy provoked because the conditions that trigger the enemy to provoke depend on which weapon is being wielded. For example, if you were wielding the pole-arm, opponents 10' away but not adjacent would provoke, and vice versa. It would be akin to allowing someone to threaten with a sheathed weapon because they have quickdraw. It doesn't matter whether or not you can take free actions because it would be out of order. (You have to be threatening an opponent in order for that opponent to provoke).
I don't think it matters much one way or the other how your DM rules, unless you are building a Combat Reflexes based Attack of Opportunity build.
I wouldn't allow it.
My logic would be: You're either wielding a two-handed reach weapon, and thus threaten your reach, or you're holding your two-handed reach weapon and are wielding a spiked gauntlet. You have to pick which of the two it is, you can't just claim to benefit from both.