You will need your own copies of the actual Infinity Engine games. Some sites where you can find them are:
You may be able to play some Total Conversion (TC) mods. Those require the original games as well, most commonly BG2.
To just try out GemRB, get the free BGII demo.
Due to low interest, it is unlikely GemRB will support EE versions of the games anytime soon, though there is already considerable feature overlap.