Martin, I would love to agree with you : - )
I know that there are several Gitter rooms. But at a first glimpse they do no look very busy: the last chat in Gitter seemed to be a month old.
And there does not seem to be a real chatting going on.
But I might have judged too fast.
Anyway, all the advantages you see in Gitter are also valid for vector.im.
But the former is not free software.
Please have a look at vector.im and check if it's not too late to revise the idea of using Gitter.
I'm ok with OTS using non free software. But if a solution pops up that has the same features (or even slightly inferior features) and is free, I really think that a switch should be seriously considered.
(Yes, using Meetup is a real issue for me! But I don't see a valid alternative yet... Openki, openki... why nobody uses you?)