Gilby was in GN'R for about three years from late 91 - late 94. He's produced and played with LA Guns, Nancy Sinatra and The Bronx. Played with Heart, MC5 and others. Been in a supergroup with Tommy Lee and Jason Newstead. Gilby's solo albums don't aim to reinvent the wheel but they're fun and well crafted. Maybe his solo stuff doesn't sell a ton to the mainstream but neither does Izzy's. Doesn't mean they're not good.
As for the the old chestnut of Gilby not being good enough to write with GN'R. Since letting Gilby go GN'R released one album.