I think Dr. Rosen might be Gary's dad.
Notice all these little hints;
Everybody calls him Dr. Rosen except for Gary's mom, who calls him Leigh. (Lee?)
Gary's dad isn't around at home.
In one of the episodes, Rosen told Cameron that divorce was always hard, and he knows what he's going through.
Therefore, I think Gary is Dr. Rosen's son. Gary's mom and rosen were once together, got divorced when gary was young, and that's why rosen got interested in Alphas.
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