There's also ElectriKeys-- free to try, cheap to buy. I haven't tried it yet myself, but the samples I listened to sounded pretty good.
Edit: If you want something free, I downloaded those GigaFont samples you mentioned and was able to play them with no trouble in SynthFont without having to convert them. SynthFont2 requires a license, but SynthFont1 is free donationware/nagware. It's a DAW, not a VST plug-in, but I guess you could use a MIDI keyboard to play a tune using CP80.gig as the sound font, save the output to a file, then load the file into your favorite DAW to add it to your project-- if you really want to go to that much trouble to avoid spending $35 for ElectriKeys.