Gourgeist's "Pumpkin Bomb" attack says to "discard any number of Pokemon Tool cards from your Pokemon", but can I choose a different Pokemon other than Gourgeist for this attack?
The term "Pokemon" in the attack is plural, so you can choose any or all of your Pokemon and discard as many Tool cards as you like from them.
Source: Crimson Invasion FAQ (2017-10-20), TPCi Rules Team (2017-11-02)

Relevant cards

Note: All applicable cards may not be listed below.
SM4_45