I'm sure there has been a few that have made that move. SWA requires 1000 turbine PIC, and the B1900 certainly counts. If they got a 737 type prior to the interview, it's fair game.That has nothing to do with paying to fly right seat at Gulfstream. Those testimonials would be a lot more valid if you actually went straight from GIA to SWA.
Either way, GoJets (your employer) sucks.