Important to know when you spend the gems: When you prestige, you loose all your parts.. so people recommend wait till prestige 10 stage 24. If you still want to spend some gems before that (for fast promotions or helping your gang in battles), read the decision making below.

• 'Super new parts Box' 300 gems: The lifter you can get from this is by far most 'gamechanging' gadget that is very good for promoting in ladders. The lifter can be used for a long time (even if its a legendary for a frame u dont use). After prestige 4 or 5 you can get lifters from normal wooden crates aswell. Other goodies are legendary shotgun or minigun, but they get 'bad' pretty fast if you promote a few stages and get a better damage 'common' weapon.
• 'Super frame Box' 300 gems: A legendary frame can go a long way, but you can easily get terrible frames only from this (legendary frames with 1 weaponslot and nothing else, 100% bonus to drill) and waste 300 gems
• 'Super parts Box' 300 gems: A legendary wheel or gadget with 100% health to a certain frame can go a long way (even stage 1 legendary parts can be used at stage 15-20). Legendary weapons u get from this are obselete fast if you're promoting.
• 'Giant Box' 150 gems: Alot of parts for only 150 gems, but no legendary. A great crate to find a new frame with enough weaponslots, a normal strong weapon or 'parts' to upgrade your parts
• 'Super Giant Box' 300 gems: Alot of superparts, 2 legendarys.

Conclusion: If you're current robot is full with parts from previous stages and you need a new build from scratch, get a Giant Box. That gives you alot of 'ok options' and will most likely give you a build to promote (and merge the stuff u dont need to level up your parts). Its proboly a bigger chance to get a usuable frame from 2 giant boxes than from 1 frame box. If you already have a decent frame, i would pick 'new parts crate' and hope for a lifter.

If you already have a good build, you can also choose not to promote in your Phase and use the gems to reset your opponents in gangbattles. Going 10/10 and using 30 gems to get another 10/10 (10th oppo gives legendary) is cheapest gain for gems, but you gotta be able to do that.