The spellgun trait appears on items that can generate a magical effect that flies out like a bullet. You can make either a spell attack roll with a spellgun or a ranged attack roll using your proficiency with simple firearms. Spellguns have a range increment, which applies regardless of which type of attack roll you make. Though you can use your modifier for simple firearms, a spellgun isn't actually a firearm. You don't get the damage of a firearm Strike, nor other benefits like critical specialization. Similarly, you can't load or reload a spellgun, affix talismans to one, make a spellgun Strike as part of an action that lets you make a firearm Strike, and so on.