I have a BAM file and I want to obtain the copy number profile of that file using HMMcopy, but I don't know how to obtain the GC content file required.

The vignette says this:

It should be noted that due to memory and speed considerations, the raw input that feeds in to this package are required to be generated by programs distributed as part of the HMMcopy Suite available at:

Which is admittedly pretty unhelpful, except it does tell you there's a thing that you can use. And googling that term brought me to this github, which seems like the thing?