The Short Version
The scientific record consistently credits Osaka University as the origin of the first GFP transgenic mice. A 1995 paper by Ikawa, Okabe, and colleagues at Osaka University is recognized by independent field reviews—including a Nature Methods history of fluorescent protein technology—as the first report of GFP-expressing transgenic mice. No credible evidence of an earlier GFP mouse from another institution exists in the available literature. The term "GFP mice" broadly encompasses many later lines, but Osaka's priority for the foundational work is well established.