No - or at least not necessarily. And that's even assuming that the IMSA licence is still "active" (in that sense; PD seems to lock down in perpetuity licences for content use, but the acquisition may be contingent on licensing and if that licence period has expired...).
Different series operate, reserve, and grant commercial rights in different ways. I have no idea if IMSA reserves all image rights of all participant vehicles and their relevant sponsors, and all circuits and their relevant sponsors or not. If so, it's a case of being able to access and scan the content, which is just logistics (but can be made difficult by reluctant bodies). If not, then simply having the IMSA licence removes one obstacle - a big one, for sure, but just one - and perhaps opens doors, but the cars and tracks would still require individual licensing agreements from the rights holders: venues, car manufacturers, race teams, sponsors.
Some series - not IMSA, by the looks of what we have - also operate an exclusive-use clause: the ability to only use relevant content in that context and no other. That'd mean you could only use the cars/tracks in those specific cases, so no racing them against other cars from elsewhere in the game, and no use of the circuit(s) for other events, which would be absolutely useless for GT.
Licensing is... very complex, and even when you think you've got a handle on it there's another layer of bull**** you wouldn't have thought of.
Why?
A lot of brand PRs see no value in getting old cars into games, because they're not selling old cars; old cars don't make them money, to their thinking. PD's processes aren't particularly rapid either, so by the time a new car that the brand is willing to licence arrives in the game it might be a facelift out of date, which would be just as pointless to a brand PR.
But again, licensing is the start of it. Once licensed they have to find an unmolested example and arrange (pay for) access to scan it. Super easy, barely an inconvenience for a CX-30, not so much for [pick a hypercar where all seven examples are in climate controlled bunkers and the squillionnaire owners know jack**** about games and couldn't care less].