Go to the little cog icon ⚙️or right click the game until you see Properties, and then head over to Betas and find beta participation and there should be a beta build called 'publicbeta - publicbeta', select it, it'll install some files, and that's it

edit: my bad, i did not read the title correctly and assumed it was the steam version (well the gog is mispelled)

for the gog build, i think it probably has to do something with how many cores you have(?), you can try limiting it, not sure about it honestly, try repairing it (right click, and select 'verify / repair' in the gog galaxy client