I used to have an Act Labs GS.. seriously, if you haven't paid him then don't. He isn't lying, but he is certainly stretching the truth (if he ever even personally tried MAME). The guns will work with MAME with mouse emulation, but they are horribly inaccurate, lose track of your calibration constantly, and can't deal with the resolutions of most old-school arcade games.
There were and probably still are people *giving* their GSs away.. I threw mine out, personally.
And about the game port..? It's slow/pokey-ness will wring the last bit of enjoyment out of the gun that you would have had playing Die Hard 2 (the only game I ever found that worked *well* with the GS).