I'd assume that top decking it or trashing it with Watchtower makes you lose track of the card, since "When you gain" happens at the same time as the card, while "play it immediately" happens just afterwards.
I'm not completely sure about the lose track rule, but I think that if you gain a Nomad Camp with a GoM, then it will play the Nomad Camp, since that's where the card "expects" the nomad camp to go. If for whatever reason you gain another card at the same time (eg if Embargo was when gain), and you top deck that above the nomad camp with watchtower, then you can't play the Nomad Camp.
If you gain a Death Cart, you play the Death Cart immediately, but not the Ruins.
If Feast is on the board then you can use this to gain $5 cards, but not to play them, obviously.
Trader means you never gain the action card, and therefore don't have an action card to play.