Save up (and Limit Break up to 3-stars, MLB is Gem expensive) all Gacha SSRs and event SSRs, and Disaster SRs. Kaisers are a whopping 10k Orbs each, so they're not particularly cheap.
You can also technically save Gem Gacha Rs and/or Gacha SRs (which sell for as much as Disaster SRs), but if you ask me, those will take too much inventory space to be worth it.
Jewel gacha is extremely unlikely to give you anything good, whileas inventory expansion will make your daily Gem Gacha much, much faster every day. At least until the next anniversary Gem Gacha update that is, which is still far, far away. As such, inventory expansion > Jewel Gacha.