Gimp is linked against X. The problems are, our X server hasn't had any love in many years. It was built with EMX and non-trivial to port to kLIBC due to it using the EMX select() (needed for pipe support, can use TCP/IP) and also some EMX internals. Some of us do have HOBX as an X server, here it only runs on Warp V4 and complains about not being licensed for eCS, others who bought eCS 1.x do have it working.
X86sup.sys also needs updating to interface with the newest kernels, missing functionality includes PTTY's being broken and a couple of other things that I forget. I did manage to install X on eCS 2.1 using the XVNC server, which allowed GIMP to run in a VNC window here. Xterms were broken due to missing functionality in x86sup.sys as mentioned. (source is available)
At one point I did build the X libraries with kLIBC (they're on the Netlabs FTP server) which allowed building with newer GCC and using LIBPATHSTRICT, and using the full IP address (to avoid pipes) to display on native X.
Starting with those libraries or porting newer ones would allow building with newer ports but still need an Xserver.
I have no idea of how well newer Xorg would build on OS/2. We used the xmakefile process that they ripped out and replaced with autotools.
So basically need a working Xserver, probably newer X libraries and then porting would hopefully be trivial as we have most other libraries already ported.
At one point we even had QT, Gnome, most window managers and even Mozilla running on X, and our Xserver could even load the native Linux ELF modules to have fully accelerated video under X which ran on its own desktop in a full screen session.
If you're interested, I'll help how ever I can.