After installing the Inkscape, let's try opening the diagram you made with GMTs.
Launch the Inkscape and select the file in the "File" → "Open".
Has the diagram been displayed on the Inkscape?
How to Edit a Diagram
Select the diagram "Objects" → "Ungroup"
You can select each part of the diagram (text and lines) individually and edit them.
How to export figures
"File" → "Save As" → ".PDF" (Save as PDF)
"File" → "Export to PNG Image" (Save as non-editable image file)
(PNG file is useful when pasting figures to PowerPoint or Word.)
You can easily find out how to use the Inkscape on the Internet. (It is also introduced on Inkscape's website.)
Familiarize yourself with GMTs and Inkscape, make beautiful figures and write fine reports!
Just because the figures and tables are pretty (they just look good), they look like fine reports.
The contents of the report are important, but the appearance is also important!