Just because Git branches are lightweight doesn't mean they MUST be lightweight and thrown away. In your use-case, the `gui` branch ref would point at the merge commit, so you could simply ask for it by name. Being an old branch, however, you could then move the branch into an 'attic' or a different namespace than the active branches so it doesn't come up every time you `git branch`. That sort of flexibility to move branches around when needed was what made me question Mercurial's approach to forever-branches in the first place.
This is but one example. On git, if there was more back and forth merging between two branches, you can't figure out on which branch a commit was made later by merely inspecting history unless you encode this information in commit messages.
