You can only activate “Chaosrider Gustaph”’s effect during your Main Phase 1 or 2.
“Chaosrider Gustaph”’s effect does not target.
Netrep Rulings:
The ATK increase by the effect of “Chaosrider Gustaph” only applies to Spell Cards specifically removed for that effect.
If the ATK of “Chaosrider Gustaph” is raised by its effect and the following turn, control of “Chaosrider Gustaph” changes to the opponent, the opponent may raise the ATK of “Chaosrider Gustaph” through its effect.