Starmie is an excellent counter to Infernape, as is Tentacruel and depending on if it has HP [Ice] or not Dragonite makes a good check. Countering Gengar completely depends on its move set. Scizor can counter the most popular Sub-Pain Split Gengar, and Blissey can counter non Focus Punch Gengar. Suicune friends can check variants without Thunderbolt. Overall, once Gengar is in play it can be tough to switch into, but it is so very frail that if you can get in a Pokémon that can take a hit, like Metagross, it is pretty much dead. Intelligent prediction is useful when facing Gengar... Like, if your T-Tar is in on a Gengar, maybe switch to Choice Scarf Rotom-A on the Focus Blast and take it out the next turn with Shadow Ball.