TDP: If you already have the game installed, and then have a "huge update", you run into the same issues with a game dev's poor and inefficient patching (e.g. uninstall/reinstall a game just to patch it, redownload large parts of the game again).
The developers don't create GOG patches, GOG does. All the developers do is provide the updated version of their game to GOG. It's because of GOG's current patching scheme that having to redownload large parts of the game ends up being necessary, because GOG offers only a single cumulative update along with the base installer. So the latest update has to include all previous updates so that it'll work on any version (back to the current base installer), and as soon as there's a huge update... all subsequent updates will be huge as well, unless the huge update is rolled into a new base installer instead.
I can understand why GOG does it this way though (it keeps things simple). It's not a real problem for me personally, but I can see why it's potentially a big issue for people with limited bandwidth etc. and I think it's something that GOG should look at & consider alternative solutions for.
But I remember many older games having alternate patches (even for the same patch version), some incremental, some cummulative, and gave that choice to the user (e.g. a smaller patch for v1.2 to v1.3, or a larger cummulative patch to v1.3). I don't know if GOG allows it at the moment, but perhaps GOG should allow the devs to upload alternate patches (giving the choice to the user). Then if there's a more efficient way to patch, the devs should make that patch available (including a direct patch from the base installer directly to the latest version).
I think a mixed approach would probably be better, providing cumulative patches but with additional, separate patches after larger updates.
e.g.:
Patch 1 - 100MB
Patch 2 - 100MB
Patch 3 - 3GB
Patch 4 - 100MB
Patch 5 - 200MB
The first 3 updates would be provided as updates to the base install. Then, due to the size of patch 3, patches 4 and up would be provided as updates to patch 3, until there's another large update at which point subsequent updates would be provided as updates to that version. And so on...
After a while you'd have to install a few updates to reach the latest version, but you wouldn't need to redownload the base installer nor would you have to download & install each individual update separately. Also, such large updates don't happen very often anyway.
GOG could even still replace the base installer with the release of the larger patches to make things easier for new users, as long as the earlier patches remain available (perhaps in a different part of the download page) to enable users of the old version to update without having to redownload the entire thing.