Our primary schools set for ability and if you are in top set, then the level is really quite good. In Year 6, the top set dd was in were doing maths at what would normally be Year 8 level.
At junior school, there was also a G&T club at lunchtime which dd was invited to join (she declined).
There were also the usual music lessons and an art club- any of those could count as stretching sideways whether you are G&T or not. Some gifted children make great progress in music. And there is an IT club.
Dd had her own library card at the local library very early on, so I can't exactly say I had to take reponsibility for stretching her: she did a lot of that on her own.
Now she is in secondary school, again there is setting for different subjects.
Also, there are all sorts of after-school clubs (though dd can't join as has to be in time to leave in the disabled taxi).
Also a lot of secondary school work is in form of projects, so those can be done to any level of difficulty: she is using books written for adults for her history projects and is doing a lot of work on her own. My only contribution is offering to get books out of the university library for her as and when needed.
SO there are all sorts of ways of getting yourself stretched sideways, even if they're not necessarily labelled G&T.