There’s an Endless Waves event launching at 0700 UTC on 21st August with a damage bonus for the looper, so it’s almost certain that there will be a Looper on offer on the special crate - either when the current Radiation Field offer ends (0700 UTC on 20th) or on the following rotation.
Unless you can afford to regularly buy gems and buy the SBP, I don’t recommend trying to buy the Looper (for reasons elaborated here).
However, if you are willing to spend upto 27,600 gems for it, then it would make sense to buy the gems while the Gem Collection event is on so you can get some bonus rewards. Hopefully the Looper will be offered before the Gem event ends, in which case you can try for it with collected gems and only purchase gems if that doesn’t work out.
Good luck 
