It seems {gdtools} is only used once as a utility to make things work on CRAN, and {extrafont} is probably not used at all - only in code that is commented out if I am looking correctly. The gdtools::set_dummy_conf() function used in {hrbrthemes} is marked as deprecated so I am not sure if it does anything.
I rely on {hrbrthemes} as a dependency (and inspiration for custom scales functions) in my personal package {ptrr} so I sometimes notice the extra demands of {gdtools} depending on Rcpp.
Just noticed in your Daily Drop that you are planning a CRAN release so I thought this might be the right time to consider.
Thank you for maintaining this!