is there anyway to determine what the market share is like for the major players, d20, WW, Gurps, etc?

Hi, Alex. Welcome to the Forge; you're practically new, are you not?

To answer your question: There's no known systematic and reliable data on this. The best ones are a WotC marketing survey from some years back and any media research done on individual retail stores (I'm thinking of that american professional magazine, name of which I forget and which publishes lists of collected sales data), which tend to be somewhat unreliable due to retailers not having a clue and concentrating on CCGs anyway. The common wisdom has it that d20 is the largest by a far margin, with WW trailing significantly and a number of third-class companies (SJG, Atlas, AEG, some I probably forget) after that. Anything below that is "small". I suggest taking any actual numbers above that with healthy suspicion.

To critique your manners: Read the stickies at the top of each forum, especially the Site Discussion one. Look at how other people act. In this case, your post is both terse (not giving us any context to give you useful advice is not a good way to begin a thread) and in the wrong forum. The correct one would be Publishing, and I guess we'll be moved there soon. This is not a big deal, but remember it for the future.