There are only a very few activities which work the body in the way that cycling does. Think of cycling as an analogue type of training, where you’re efforts work on a constant wave, of increasing and decreasing efforts, with peaks and troughs, and plateaus. Another activity that has this profile, is rowing, but rowing uses a lot more upper body, than cycling. Doing general Gym stuff, is digital, you’re either exerting, or not, a bit like switching on or off, with nothing in between, the two types of exertion really aren’t complimentary. Strength gained doing weights / resistance stuff, is actually pretty pointless, if you want to be a better cyclist. Cycling is more about muscle endurance, than outright muscle strength. If you can stand on one leg, then the other, you have enough leg strength to cycle, you don’t need much more. Doing other physical activities ( like general gym stuff) won’t hurt, you’ll be working your CV system, but it won’t help improve your cycling much if at all either. You’d need to be doing targeted drills, on the more cycle specific muscle groups, for that to work.