As I understand it, GMT.jl is a Julia wrapper around GMT which is written in C / C++ and uses PostScript (Generic Mapping Tools - Wikipedia).
I am relatively new to Julia. What I found is that the Julia Plotting World is very colorful and even packages like Gadfly and Makie that I understand are written fully in Julia are handling the same things slightly different. So in general, you have to familiarize yourself with each package and learn some of their intricacies. As a side not, you may also take a look at Plots.jl which is doing some kind of “integration” over various plotting backends, and is providing a common interface.
I do not know if GMT is better or worse to above mentioned packages, what I think is that it is handling things very different to Gadfly and Makie under the hood as I understand due to its C / C++ origin. As for PostScipt, some time ago, you could hardly go any better than using it in the professional printing world, however, this might have changed over the recent years. Even though, I guess PostScript might offer some advantages. Please see How to create layouts in Julia plotting packages? - #11 by joa-quim
Im not sure if I fully understand what you want to do. As for know I am assuming you want to save a plot. If so, I understand the option savefig="figname" should be working: What is your favorite mapping package? - #22 by joa-quim
EDIT: I just spotted the question was answered by @joa-quim