You've been working in these forums long enough to know that telling us something like:
without showing us the diagnostics produced (if there are any) of the results you are getting (if there are any) makes it hard for us to help you.

Your description of what du -sh | grep '[GM]' does on a Linux system is not correct unless the directory in which you run it contains no directories. Are there any directories in the directory where you want to run these commands? Are there any sparse files in the directory where you want to run these commands? Do you not care about Terabyte sized files?

And du does not give you file system details; it gives you filesystem hierarchy disk block allocation counts. (Where the filesystem hierarchies can be single files, single directories, or multiple nested filesystems depending on the operands you give it to process.)

Would the command ls -lSd give you the data you need and would a shell or awk arithmetic comparison on field 5 of the ls output to select the file sizes you want from the output be sufficient?