Unfortunately as it turns out segfaulting is a common iked reaction to invalid configurations (at least for invalid transforms), so what you found is a rather systematic problem (and has been on my list of things to fix for some time).
As to why those with [ESP only] trigger this behaviour: [ESP only] means the transform type can only be used for ESP SAs (meaning Child SAs), what you're specifying in ikesa is the IKE SA. Try using AES-256-GCM in the childsa option and it will work as intended. Maybe we should also change the man page to make this clearer?
