Gitter didn't have a proper Android client for years - so most abandoned it.
Furthermore in the past we had problems moderating gitter channels.
Nightscout support is already at discord, the androidaps omnipod development happened/happens there and the #androidaps channel there is quite active. It is easy to jump into calls with other devs, testers.
Between GitHub, Slack, Facebook, discord, gitter, ... there is just too much fragmentation.