I feel that HGSS had improved on Gym Leaders from GSC. There were only three back in 2nd gen in the Johto region who did anything notable - Jasmine was one with her staying at the Lighthouse to care for the sick Pokemon, which was a pretty nice thing. A pity that the other two examples were Whitney (crying upon losing) and Clair) refusing to accept defeat and making you perform an additional challenge)... not a great example they were setting. The others were cooped in their Gyms.
HGSS however gave each of them appearances outside of their Gyms postgame. You can rebattle them, call them to get some new info about their lives, or even have them call you to say something. That's on top of a few other appearances (photos for getting their number, other interactions e.g. Janine and Falkner). And even before the E4 Whitney shows off the Pokeathlon to you.
I feel that compared to that, theSinnoh Gym Leaders don't actually have much more screen time overall. Certainly there may be more who show up before the E4 outside their Gyms (Maylene does in the snow, Gardenia tells you about spooky ghosts in the mansion, CRASHER WAKE in Platinum with the bomb, and Volkner being all angsty while abusing the city's energy supply.) Hearthome's Gym Leader does do contests as well I suppose, although that's a lot of contests before you would be able to fight her in that format. But all up they don't differ too much. Mind you, HGSS did include some Sinnoh Gym Leaders too (while DPPt only had Jasmine).
Gotta side with the HGSS Gym Leaders though, but that's partly nostalgia talking. DPPt did do Gym Leaders better than any previous game by far - they felt more like real characters with more to do, and only fall behind BW in that regard. (A shame XY went backwards with this...)