I know of the obvious things such as purchasing another builder or a shield. I know I can boost production of gold and elixir. I know I can use gems in order to make up for a shortage of resources when purchasing upgrades. Is this all the possible things you can do with gems or is there something more?
There are a bunch of things that gems do for you. Most are related to the reduction of timers or bypassing resource requirements.
- Buying builders can only be done with gems, which you've already mentioned. There is a max of 5 builders, with each builder costing more gems than the last
- Buying shields can be done with gems, although you can get them from being attacked as well
- You can boost unit production at barracks with gems
- You can boost spell creation at the spell factory with gems
- You can boost the production rate of collectors (gold, elixir, dark elixir) with gems
- You can heal your heroes with gems (bypassing the timer)
- You can make up for a resource shortage when beginning to build/upgrade/research/train something with gems
- You can also bypass wait times on build/upgrade/research/train processes with gems
As far as gem acquisition and strategy is concerned, I wrote a pretty popular article on the subject, which may be of interest.