Ah! Yes. ARM64 is not binary compatible with x86. GIMP Installer contains binary images for both architectures; the installer detects the architecture of your Surface device (ARM64 for you) and installs the correct binary. So — I imagine that GIMP 3.0.2 runs on your Surface machine without G’MIC being installed because the GiMP team is providing ARM64 binaries.
@David_Tschumperle may correct me, but I believe that the G’MIC plug-in for GIMP on Windows is only built for x86, and that binary would not run on an ARM64-based machine. Qualcomm Snapdragon X CPUs on the newer Microsoft Surface machines is still new to G’MIC; I don’t think we officially support it. We are a very small team and I think it is David — just David — is the only person providing the binaries for the Windows install on the G’MIC download page. He will have to weigh in to say if ARM64 is within reach (or, that I’m wrong, he is already providing them). If I am not wrong, and that, indeed, we only provide x86 binaries at the moment, then that is your problem: our G’MIC binaries are x86, not ARM64, and you cannot run them.
I suppose, if that is the case, we should post an advisory on the download page that G’MIC currently provides only for x86, Intel or AMD processors. (Windows users: Click on “Settings”. Select “System”, then “About.” Look at “System Type” under “Device Specifications.”, It will indicate either “x64-based processor” (for a 64-bit Intel or AMD chip) or “ARM-based processor” (for an ARM64 chip). G’MIC binaries do not run under ARM64).
Thank you for reporting!