@reginald: First ghutchis could add you (using your WPORG username) as a contributor & committer. That would a good way to indicate to everyone that this is part of an agreed handover.
Next, you need to establish if ghutchis wishes to retain any involvement (albeit just on “paper”) with WP Gallery. If “yes”, then neither of you need to do anything more. WP Gallery can continue “as is” with Reginald being the more active developer.
If “no”, then (due to the technical limitations of the WPORG svn system), Reginald would need to ask for a new account for a “new” plugin called (say) WP Gallery Reloaded. Once that has been established and the new forked plugin released, ghutchis can ask for the original plugin to be removed.
Does that all make sense?