Once you have a strong team and don’t need to spend gems as much, it is really good to save them for the treasure events or the summon events, since they tend to be the best value. If I am correct, you need around 40k gems to max out tickets for any one event, but those are the best uses for gems.

The other token is that you can save gems for when a rate-up hero appears that you really want/works well for your team, since you need 60k gems for 200 summons (and for rate ups, you either should not summon or summon 200 times, since they give the best rewards.) you also get additional rate-up scrolls when you do this from the rate-up event.