RG fan since 2012 here, I had the same question when I resumed watching RG actively in 2018 after life settled down. The short answer is that while there are/were a few non-English RG forums (e.g. Russian rg4u.clan.su, Italian Beatrice Vivaldi (closed)) an English RG forum is has not been in existence for at least 7 years, fans are on social media like twitter and instagram or on Telegram chats but there isn't a site that acts as an international fandom hub as GS does with FS. Probably because unlike AG (or to be precise, WAG), RG is far from major in the US (or any other anglophone country for that matter) so there isn't enough demand for people to gather and run a forum. Like, I can see a Spanish-speaking RG forum coming in existence (a lot of RG fans on twitter are Spanish speakers, including the ones who archive videos/run streams/etc. and thus have a following) but I don't see an English-speaking RG forum happening any time soon. Maybe if something miraculous happens, for instance, my US fave Rin Keys winning Olympic gold, then maybe, but as much as I adore Rin it is unlikely to happen so
There is already a RG thread on GS (
https://www.goldenskate.com/forum/threads/rhythmic-gymnastics.98805/), made in time for Paris 2024, but as you can see not much people were on it, and it was the Olympics. If I start discussing WC Sofia soon to take place in Bulgaria there, I'll probably be talking to myself. It's sad but it is what it is.