Since GIM is a 'less arduous' play type than IM, I don't see a problem with this.
You're allowed to 'step down' the self imposed challenges (at least in RS3 you are which is what this subforum is about) from HCIM to IM and from IM to normal. GIM is basically in between IM and normal. So it makes sense to me.
I will point out though, you may find that starting over makes all parties involved feel more equal and evenly matched for inter group competition, if you have friends who will be making an account specifically to play GIM. If it's just you and several others who all want to transfer your irons into one group together then this wouldn't apply.
11-Oct-2021 14:33:27