If I use Gung ho on the Sentinels do I have to heal the one that uses a power? Or could I heal Mainstay and use Medico's power.
I'd say it works like Embolden does. The consesus on this thread was that the targeted hero takes the damage, but any power may be used.
The reasoning is that while targets take damage (and get healed), heros use powers. The sentinels are one hero (although 4 hero targets), which is why they only have one deck, one trash, and one hand. So although each power is on a seperate character card, the Sentinels collectively have the four powers - they aren't individual.
Personally, I'd play it as you heal the one thats going to use the power.
But I do not have a strong feeling about it either way. If you wanted to heal Mainstay then use Medico's power, I think that still stays within the intent of the power.
However, Infection says it plays on "one hero", and while the Sentinels count as one hero for the purposes of Rat flipping, only one of the four is the one who's actually Infected (and thus has to hit themselves). We've been playing it the same for Kismet's Jinxes as well, if they're the applicable targets, wherein only one member of the Sentinels actually has the Jinx put on them if it's possible to do so (for example, if they are the "hero" with the most cards in play, in which case they all qualify, at least at the beginning of the game before someone else has had a chance to outnumber them with cards in play ;)).
I think when we played with Greatest Legacy and the Sentinels (only once so far, I think), we decided that both parts of the power would apply to the same member of the Sentinels...which I think was fine at the time since the Idealist would've been the target for both anyway, having taken more damage (from stuff hitting the lowest hp) and being the one to use a power to cause Sentinels Tactics to trigger and let one of the other three do something too :).