I'm sure everyone's familiar with Unknown G and it's power "Guard." My question is, is added damage (like damage with a + next to it) count as an effect of an attack and is therefore blocked? Or does it go through because it is damage?
It goes through usually if it's a coinflip to do more damage or something along those lines. It mainly prevents Status Conditions, placing Damage Counters (different from damage), and bringing up stuff Active from attacks.
So, like, Leafeon X's Verdant dance doing 30 damage plus 10 more damage for each energy would do, say, 60 damage if leafeon had 3 energies even if the defending pokémon had unknown G, right?
Unown G GUARD blocks all effects of attacks, excluding damage. Damage is the only thing it doesn't block, so anything that does excess damage, such as Verdant Dance or LA-Gigas' Gigaton Punch, will not be negated by Unown G, as it's still damage.
It blocks status conditions, damage counter placing, and any other effects of attacks done to the equipped. Toxicroak G and Bibarel from Stormfront have Poke-Bodies with identical effects.