At the very least, that's an issue that GMP or other projects shouldn't have to worry about. There are many options - you could manually cache things somewhere or pack in the dependencies in the repo. Or, maybe, in a world that wasn't completely set on wasting all resources possible, there just wouldn't be pointless automatic builds on forks, and those builds wouldn't need to re-download the world and could instead just incrementally update. (yes, there are nice consequences of doing fresh builds always, but there are also bad ones as can be seen, and unfortunately the downsides aren't seen by the initiator)
