Polyphony released 3 full GT games between 1998 (or dec 1997 in Japan) and 2001.
But what about Square?
FFVII 1997
FFVIII 1999
FFIX 2000
And not only this, but Square also released PE, Xenogears, Chrono Cross, Vagrant story, SaGa Frontier 1 and 2 and more (I know I skip some legendary games now) inbetween these 3. Square's PS1 run is to me the best third party run of all time.
Its just crazy how fast games surfaced back then, though i am aware todays games are far bigger and usually extended with DLC.