How to build / ideas for a multiplayer lifegain control deck?
There are a couple main reasons you might see a lot of multiplayer decks with 1 or 2 of each card.

First and foremost, multiplayer is a pretty casual format, so people are playing with the cards they own, and trying not to spend all their money. They probably only have 1 or 2 of a lot of cards, so they're just building the deck they can build.

Along with that, casual games tend to be about fun, not just winning, and a lot of people do like the excitement and variety of having a lot of different cards. There's something to be said for getting to cast a bunch of different spells, even if they're not quite as good as the 4-ofs you might put in the deck if you wanted to perfect it. People obviously really like this; Commander is getting pretty popular, and the entire format is built around the idea of having only one of each card.

As for your specific deck idea, sure, it's possible!

First of all, remember, lifegain is in general a weaker mechanic than it seems at first, because unless you actually deal with the things that keep making your life go down, it only buys you a little time. So you really need to make it count - find the most efficient life gain you can. This means both getting the most life you can out of it, and hopefully getting an extra benefit. Happily, extort and Gray Merchant (mentioned in the comments) do both. They gain life for each opponent, so lots of lifegain. But they also drain your opponent, and give you an extra creature. (Or in the case of Blind Obedience, slow down aggro decks.)

Depending on your constraints (and your collection) there are probably a lot more cards in that vein - try searching for "each opponent" gain life or even just "gain life equal to", and you'll find some more potential candidates.

Beyond that, it does depend on the kinds of decks your friends play. If you're surviving to the late game but still losing, maybe you just need better win conditions. If you're getting killed early before you can get your cool stuff online, maybe you need some faster removal, or in general faster answers to the kinds of things they're trying to do. The control side of your deck needs to be strong and efficient too, not just the lifegain. But it's hard to give terribly specific advice here; we don't know what kind of decks you're playing against, or what kind of cards you have at your disposal. I think about the best general advice is to look at what's killing you and figure out ways to address it efficiently.