Do the poisons stack? Is it worth it? Do the healing debuffs stack?

The healing debuffs won't stack.

The only reason to get Exec Calling on Gangplank is for multi-managing healing debuffs. Or a ranged healing debuff using Parrrley. Which is good, because if you're against a Soraka and some other person who will make you pay for getting in range of Soraka, that Parrrley will help you harrass more, since Soraka's heals won't do much at all.

Ahhh, so Parlay procs that? Excellent, thanks for the info.

Yeah, exec is really good on him. It takes the place of one of your averice blades if you choose to build it though, but it adds lifesteal and a good DoT that I believe is applied to your parley shot.

It is also very useful on Twitch, anything that allows you to prevent healing while still poisoning = Good.

Malady + Executioners = Nice combo imo.