Erm, Objective-C probably isn't the problem, even NeXt used gcc for that (not sure how much of the current feature bloat isn't in copyrighted libraries).
But GNUStep isn't exactly up to par with Cocoa, never mind not exactly the default desktop environment. Porting the whole editor from Obj-C++/Cocoa to C++/KDE or C/Gtk would be a pretty huge task, where you'd better off starting from scratch anyway (as with a lot of GUI apps, it's mostly about the ideas, not the implementation).
Don't quote me on this but I don't think the later versions of Obj-C run on gcc, but llvm/clang. Whilst thats available outside the Mac realm it depends on whether it's specifically modified.
But yeah, you're right the Cocoa aspects are the bigger overhead, I should have been more clear.
Cocotron[1] attempts to be up to part with Cocoa though. I'm not sure if it would cover everything needed by TM, but it would be a pretty decent start.
Considering that the only reason GNUStep isn't considered a totally failed project is the existence of Hurd, I wouldn't bet my money on creating a reasonably up-to-date cross-platform Cocoa/OpenStep clone. Try to see if the base Objective-C++ is working alright and create a GUI-independent layer, at least that way you'll get your native look-and-feel for free. But looking at the code, I think that'll excisce a lot of the core TM tech, so whether it's worth the conversion effort instead of just starting brand new is a pretty good question.
Browsing through the code, Objective-C seems to be used for the UI, but the guts of the code (the Frameworks directory) is in C++.
I have mixed feelings about TextMate going open source. If QuickSilver is any indication, it will now definitely take forever for a new release. Dang! and I seriously don't like ST2.
I know and it's great, but from Alcor released QS as open source until it was picked up and new proper releases started to emerge, it took 2-3 years? So waiting 6 + 2 years (in the best case) for TM2 is too long even for me. RIP TM2.
Define "new release," because Quicksilver's core app and plugins are being updated rather frequently now for bugfixes. As far as adding new functionality, that might be right but what else do you need core QS to do? It would be nice to have an additional plugin or two but as far as core QS, I can't think of anything it doesn't do that I would like it to.
If TM isn't updated much, then it's because of the same reason that QS is not updated much anymore, because most people moved on to something else and are no longer interested in it.
It's better than nothing. The lack of activity means the author isn't very interested in continuing the project. But you can find another lead developer to run the project, at least. That's the merit of open-source.
But GNUStep isn't exactly up to par with Cocoa, never mind not exactly the default desktop environment. Porting the whole editor from Obj-C++/Cocoa to C++/KDE or C/Gtk would be a pretty huge task, where you'd better off starting from scratch anyway (as with a lot of GUI apps, it's mostly about the ideas, not the implementation).