Unfortunately, the rules don’t explicitly define the terms they end up using when it comes to shields, especially when used as weapons. There are a number of ambiguities, in both Pathfinder and the 3.x rules that they derive from, around their use.
The way the rules end up working, shields can have a number of “modifier” terms associated with them. There are types, e.g. buckler/light/heavy/tower, there are materials, e.g. wooden/steel, and there is whether or not the item has spikes, spiked/(not, unmentioned).
But none of this is made explicit in the rules, which means I am basing this purely on my own extrapolation from the rules that do exist and how they seem to interact (or not) with these terms. I feel that this understanding is consistent with how the rules are written, and also matches what appears, at least to me, to be the authors’ intent, but it is impossible to give a hard-and-fast certain answer here.
In any given place within the rules where shields are referred to and certain categories of modifier are not mentioned, they consistently seem to imply that the unmentioned modifiers are not important and any value for them is acceptable.
For example, in the weapon listings, there is no reference given to the material modifier; wooden and steel shields of a given type are the same, and thus no distinction is made between them. On the other hand, distinctions based on type and on whether or not spikes are present are important, so there are separate entries for light and heavy and for spiked or not (total of four entries, one for each combination).
In contrast, for their armor properties, whether or not the shield has spikes makes no difference, but the material is important for determining weight and cost. Thus, there are entries for light and heavy and for wooden and steel, plus bucklers and tower shields which don’t seem to allow alternate materials, so six combinations, but there are not separate entries for spiked versions (though shield spikes are listed under the options section).
So when the bashing property does not specify spiked or unspiked status, it is saying that it doesn’t care about that property. It only cares that the “type” is light or heavy; whether it’s wooden or steel, spiked or unspiked, don’t matter.
Thus a +1 bashing light steel spiked shield made for a Medium creature would deal damage as if made for a Gargantuan creature (and also would have a minimum of a +1 enhancement bonus). This does make for a fairly-high base damage, but considering that shields are not two-handed and cannot have reach, they make fairly mediocre weapons overall. Base damage is a small factor in overall damage.
Also, note that bashing is a shield property, not a weapon property. That means the +1 in the +1 bashing shield is an enhancement bonus to the shield’s AC, not to the shield bash’s attack or damage values. It counts as a +2-equivalent armor for pricing purposes, i.e. 4,000 gp, not the 8,000 gp of a +2-equivalent weapon. Most importantly, you can magically enhance a shield/its spikes as a weapon in addition to its properties as a shield, so you could have a +2 bashing light steel shield that has a +1 flaming light shield bash. It would count as Gargantuan for its base damage, have a +1 enhancement bonus to attack and damage (not +2), and would add +1d6 fire damage to its attacks. When not used as a weapon, it would have a +2 enhancement bonus to its shield bonus, for a total of +3 shield AC.
For simplicity, here is a table of the damage dealt by bashing shields, assuming no enhancement to the shield-as-a-weapon and ignoring other bonuses to damage (Strength, etc.).
│ Shield │ Damage │
│ +1 bashing light shield │ 1d6+1 │
│ +1 bashing heavy shield │ 1d8+1 │
│ +1 bashing light spiked shield │ 1d8+1 │
│ +1 bashing heavy spiked shield │ 2d6+1 │