I will do some looking around but I am out of my knowledge area. You might want to start a new post with a title "blah, blah, missing font ....GGally"
In any case it definitely looks like it is an R problem not an RStudio problem.
Any idea what the font was? IIRC, if it was a pdf output it should have been a Latex font unless you specified a font. If it was html I think it defaults to Helvehica in the text. But no idea what GGally uses as a default. You might want to e-mail the author or maintainer and ask.
I think R has a GUI, it does for me in Ubuntu 20.10. It should be in the list of applications.
Sorry I am not more help.