First of all welcome to PS3news.
Anyways this is what I would do if I was you.
1. Back up a game to your Internal HDD using multiMAN manager.
2. See where the game (you just backed up) is stored and put more games in that directory so it looks something like this: "dev_hdd0/GAMES/BLES12345/PS3_GAME/USRDIR/Eboot.bin".
3. Go back to multiman and refresh the game list (press "triangle" and choose "refresh list") and the game should show.
I wouldn't recommend using Gaia Manager since it's kind of outdated. I recommend using multiMAN manager or if you want something simple use Rogero Manager (http://www.ps3news.com/forums/ps3-ha...tml#post380479
) which has the same game compatibility as multiMAN but doesn't have all the features multiMAN does.
P.S. The reason Open Manager and Backup manager don't work is because it was signed with the 3.41 keys (for 3.41 Jailbreak) to begin with and when 3.55 CFW came out it didn't work anymore. Anyways they aren't any good now compared to multiMAN.
If you have any more questions don't hesitate asking, I'm sure me or any other of our members will be able to assist you.