This is the explaination I got from the creator:
#Help building core for Gamestick 4K Lite (I have a working toolchain)
magic osprey
[14:15]luca: anyway, once you have your docker image (should be included on Github) you have to compile the sdk, for that follow the readme.txt included (section 2, compile SDK, it's not hard but it might take long)
[14:16]luca: after that you'll have a working toolchain, next step is to download retroarch and its core (there are instructions on the retroarch repo, it's a matter of cloning the container repo and running a script they provide which will download many cores)
[14:18]luca: once you cloned retroarch launch a docker container with the toolchain image including the root of retroarch core (something like 'docker run -it --rm -v /home/USER/stick/HiSTBLinuxV100R005C00SPC050:/linux -v $(pwd):/retro stick_dev bash' in the root directory of retroarch)
[14:18]luca: then, 'cd /linux', '. ./env.sh' (it's cumbersome, just do it: it will set some path variables for the toolchain)
[14:18]luca: and after that you can cd into /retro/the_core_you_want_to_build and try making it
[14:19]luca: the process is almost automatic for main cores (that's the reason why you have so many cores in gstickos and not others)
[14:20]luca: for the specific core you have requested (gsp, right?) i did this to make it: 'CC=arm-histbv310-linux-gcc LD=arm-histbv310-linux-gcc CXX=arm-histbv310-linux-gcc make platform=classic-armv7-a7'
[14:21]luca: that's not a general command but usually what i do is prepend "CC" and "LD" and try a make without anything else: if, like in this cas, doesn't work, I'll dig further into the error to see what's wrong. In this case it was a matter of passing platform=classic-armv7-a7 to make and including CXX, but it may be different for other cores
[14:22]luca: strictly speaking this is not the best way to do things, a well built toolchain should have all the required binaries ready etc, but once I had a working version I didn't bother polishing it more (yes, I'm lazy and yes I don't have so much spare time)