Does the Archery buff work for guns, too?

No. Potions, buffs, and items that improve "arrow" damage only affect weapons that shoot arrows. If you use guns, you'll want to be looking for things that affect "ranged" damage.

Don't worry, though! Despite the fact that there are no potions available that buff gun damage, guns are still an incredibly powerful weapon choice, especially with complementary gear to back them up, like the Necro Armor.