Does Gamersgate have a client like Steam? DRM? Can you redownload games again later?
Yes, you can re-download games later.
GG doesn't have a client-program like those other two (Origin and Steam).
I'm going to give details on how these work, so you might wanna read this stuff...
GG (and some other programs) do some funky things w/ downloads - i.e. they try to kill some (usually your setup.exe) or all of your installer files, if you don't back them up before-hand.
About GG - everything's done through your web-browser (Firefox, Comodo, IE, whatever, etc).
For Origin and Steam games -> Just sign into your GG account; check your Shelf for the game-keys; you can just take your Origin and Steam keys; and activate them directly through their respective game-services. It's just easiest for games of those kind of games to deal w/ Origin or Steam over GG.
For Non-Origin and Non-Steam games -> you hop on GG website; sign into your account; go to the left pane to click on your Shelf (which you can sort and whatnot); pick the games you own and want to download; download the "Download [insert game name].exe" file; run that file so it does the download of installer files.
That "Download [insert game name].exe" - it stays on top likes it is its own program (and also runs as a process in the background; like the other game-clients I will mention below) do. They're encrypted - so be careful w/ this.
Now, you need to know this -> Gamersgate, GameFly, UPlay, and G4WL all work similar in one regard: they try to destroy all or some of your installer files (G4WL kills the entire thing; the others kill your installer Setup.EXE file), once you decide to click "install" the game.
Please note where you put the installer folder. You'll need this location.
When you download a game from UPlay, GamersGate, or GameFly game, they attach a game-number to the installer-folder. You might want to make a note of your game number for later, just in case you need it later for whatever reason.
You have a few options now.
1. You might want to make a copy of the Setup.EXE file, just in case; and click "install" and let the game install; let it finish and destroy original setup.exe; select option to keep your files; and you could always rename the copy of setup.exe to its original name; rename game folder to something else.
2. OR, you could just copy the entire game folder somewhere; and rename that folder to something else; keep that as your backup; just run Setup.exe whenever you want to reinstall.
EDIT:
The only digital sites I've used for games have been GOG and Steam (and an Amazon purchase that gave me a Steam key).
Amazon's download service (for Non-Steam games) is much easier to deal w/ than say GG, UPlay, G4WL, and GameFly.
Amazon doesn't have an encrypted file running that tries to murder your setup.exe (like GG, UPlay, G4WL, and Gamefly do) or game-installer folder (like G4WL), after you finished installing your game.