To all saying that GPL content is available through conversions in modern platforms - the problem is it just isn't, no matter how you look at it.
Cars? Apart from recent version of Bazza's '67 mod, which is quite good for what it is, and some official single cars scattered here and there in AC, PC, PC2 & AMS, most of the cars in GPL mods are not available in any modern sim platform, and no modern platform allows me to recreate a historic race with correct late '60s F1 or Sport Car starting grid on historically correct track (and I don't even want absolutely full correct starting grid, 2/3 of it would be sufficient already). Not to mention recreating the whole season. Only heavily modded GPL does.
Tracks? If one want's to count in all the half-arsed conversions from conversions from conversions, with surface 3D meshes, which seem to be made with an axe and hammer, coupled with textures pull out of dog's throat, then yes, there seem to be many track available. But the fact that they actually look and feel worse in AC than in GPL makes you ask a question - what was the "modder" thinking (apart form quantity over quality rush, which seems to be the norm in simracing modding nowadays)? There are literally two hands full of great quality GPL track conversions to modern platforms (or scratch made ones) on the entire internet. Some of the other, latest GPL track releases don't even have their counterparts on any modern platforms (Riverside '66 and Road America '71 come to mind as examples).
I still do drive modded GPL occasionally simply because I'm strictly a content guy rather than gfx fidelity guy. I have zero interest in modern motorsport and all the interest in '60s motorsport, so I don't own either ACC or RF2 or PC2. I cringe a bit at GPL's DirectX7 graphics and physics limitations (theres only so much one can squeeze out of it with mods) and I would sell my kidney to have all the GPL mods content ported over to one of these newer platforms, but after playing and modding AC for a few years I already know it's just never going to happen.
So back to GPL it is, for my Can Am '66 or Can Am '71 fix for example, because there's no other place to go to.