Hi,
For troubles like this the best is to use the issues in the GMT.jl github, or even better, the GMT forum.
It’s not clear to me what is happening but things point to a ghostscript trouble.
GMT produces only postscript, and when users ask for a different format (png here), the ps file is converted via a ghostscript call. The default ps file is always written in /tmp/GMTjl_tmp.ps. Does that file display correctly?