Airforce Pararescuemen are among the best trained of all emergency teams in the U.S. military. That along with the fact that he's been involved in and survived several "off the books" operations that were vital, brutal and successful mark Gywnn as being very good at his job. Maybe too good for his own good. The powers that be decided that the PJ skill set is one that needed to be added to the first contact SG teams and orders were cut sending Technical Sargent Gwynn to Colorado Springs for evaluation and assignment by the SGC's C.O., General George Hammond.
Gwynn had ended up on SG-17 with Major Lorne as his team lead. Lorne was the one that got his team in and out of some of the nastiest fighting of the Ori war. And it was Lorne who tracked Gwynn down and told him about the Atlantis expedition.
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