If my opponent's Active Pokemon is Cetitan ex with the "Snow Camouflage" Ability, can I use Team Rocket's Giovanni to switch one of their benched Pokemon with it?

If your opponent's chosen Benched Pokemon is Cetitan ex, then Team Rocket's Giovanni's effect will be blocked. But choosing a Benched Pokemon without "Snow Camouflage" will cause it to switch with the Active Cetitan ex because the Benched Pokemon is the target of Team Rocket's Giovanni. You will still switch your Team Rocket Pokemon either way.

Relevant cards

SV10_65

SV10_174