If a magic weapon (+1 or more) leaves the hand free, it's still possible for an enchanter to use in that hand a rod of metamagic? The case is this one: the character it's equipped with a [gauntlet] and wants a metamagic rod in that hand, the doubt it's: it's not possible to have two magic items in the same slot (weapon, in this case), but the typology of items are different
I am going to break this down into separate pieces.
it's not possible to have two magic items in the same slot
This is correct, but
(weapon, in this case)
this is not, because there is no such thing as a weapon “slot.” What there is instead is the number of hands free that a given weapon requires. Someone with two hands cannot wield a guisarme and a falchion at the same time, because both require two hands, and the character only has two. The magic on these weapons doesn’t matter at all. Even if both are non-magic, they still can’t be used together.
A metamagic rod also requires a free hand. You could not use a longsword and a metamagic rod in the same hand, because they both use that hand. This is, again, true even if the longsword has no magic. You can probably (with DM approval) even carry a +1 longsword in the same hand as a metamagic rod—you won’t be able to use the sword like that, but a person’s hand should be able to wrap around both securely enough to hold onto them. And the magic won’t interfere with one another, and the rod will still be usable (because magic rods only need to be carried, not “wielded,” to be used).
If a magic weapon (+1 or more) leaves the hand free, it's still possible for an enchanter to use in that hand a rod of metamagic?
Weapons that leave the hands free are unusual. There are armor spikes, which are totally separated from the hands and seem unrelated to this question. There’s the unarmed strike (which can be a kick or headbutt or whatever), but since you can’t (usually) put magic on that, it doesn’t fit in this question either. And so that leaves the gauntlet, and maybe the spiked gauntlet and claw bracer and perhaps some other items.
The gauntlet, though, that one is clear: you can wield a weapon in a hand wearing a gauntlet. And since there is no weapon slot—just the use of free hands—we know that the magic of a +1 gauntlet does not interfere with the magic of a +1 longsword. There is no worry that they are both weapons, because there is no “weapon slot.” The magic is fine.
But just because the magic is fine doesn’t necessarily mean you can use both. The rules don’t say, so it’s up to the DM. Depending on the design of the sword, it might be impossible to punch with a gauntlet while wielding the sword—because you’d hit with the guard or pommel or something, and maybe that doesn’t count? We know, of course, that such things were certainly done in real life, but we don’t have rules for them. Again, the magic doesn’t matter here. The magic is fine, but we might have a problem even if both items are non-magical.
That is going to have to be up to your DM, but it gets harder and harder to justify not allowing it as you get to weapons with less in the way. A sword might have a guard covering the hand, but a flail doesn’t. And a metamagic rod certainly doesn’t. It shouldn’t be too hard to punch someone with that in your hand. And we know people can punch just fine with their hand wrapped around something—plenty of actual weapons (e.g. brass knuckles) require exactly that.
So it’s up to your DM, but it should probably be allowed.
The case is this one: the character it's equipped with a Claw bracer and wants a metamagic rod in that hand
This is ever-so-slightly trickier, because Arms & Equipment Guide says this about the claw bracer:
The wearer can cast spells normally while wearing the bracer
Can cast spells—but maybe, technically, not do anything else that requires a free hand, like carry a metamagic rod. How a hand free for performing complex somatic gestures could somehow be unable to simply grasp a rod is unclear to me, and I would certainly allow it, but technically Arms & Equipment Guide doesn’t say you can, which means you should ask your DM before you try to do so.
Spiked gauntlets also have problems, for the record. Because spiked gauntlets are defined as light weapons—not unarmed weapons—they arguably fill the hand that wears them, and that hand can’t be used for anything else, just as a hand that wields a dagger can’t be used for anything else. This doesn’t really make much sense at all, but you’ll have to check with your DM before you use one this way.
Basically, gauntlets are fine, claw bracers and spiked gauntlets should be fine but double-check with your DM because the rules don’t clearly say it’s fine.