GAIN can recognize when a channel already exists in another brand and automatically imports it. To move an existing channel after a new brand is created, simply click on Add Channel in the new brand and GAIN will recognize it.
All the data such as posts and approvals will be transferred.
After the channel has moved, make sure to verify in Account People that everyone who has access to the channel in the old brand have access now. Also, approval templates don't move so create the appropriate approval templates on the new brand.