The only way to change the parameters of a system action is to make a copy of it. I don't know why you'd have to give up the gallery. I suggest just leave it as the first accessible tab. If you enable the stream system action you'll have an activity stream as the first tab followed by gallery which fits a group structure better. Most of the GJ integrations won't exist anymore in GJs next major release as they'll be replaced by CB Activity and CB Gallery integrations. My recommendation is the following for full featured groups.

Publish "CB Groupjive - Stream" System Action (or your own copy)
Publish "CB Groupjive - Gallery" System Action (or your own copy)
Publish CB GroupJive Events (if you need events)
Publish CB GroupJive Forums (if you want Kunena integration)

This will give the following tab order.

Activity
Gallery
Events
Forums
Users
Invites

You will not need the following integrations if using the above.

CB GroupJive Wall (replaced by "CB Groupjive - Stream" System Action)
CB GroupJive Photo (replaced by "CB Groupjive - Gallery" System Action)
CB GroupJive Video (replaced by "CB Groupjive - Gallery" System Action)
CB GroupJive File (replaced by "CB Groupjive - Gallery" System Action)

This will become the default setup in a future release once the above 4 integrations are removed.