It's not clear to me how GtkFileChooserDialog relates to the question, since "bookmarked folders" would seem to be an OS-level (or at least window-manager-level) feature, not something specific to an API call for a GUI library. Do you mean that you used a program that generates a file-save or file-load dialog created by GtkFileChooserDialog, and used that dialog to create some bookmarks? Or that you are writing code to generate such a dialog and want it to populate GUI fields based on some bookmarks? Or what exactly?

"bookmarked folders" would seem to be an OS-level (or at least window-manager-level) feature

Why?

... Because they persist between uses of the dialog? And because, at least in Nemo, I can see bookmarks in the left-hand panel of the file browser windows? If you had a different kind of bookmark in mind, then I don't understand what it is and the question would probably be better of with an explicit example.

Based on the answer that "worked for" you, it seems that I did have the right "bookmarks" in mind. You can create these in other ways, for example by using ctrl-D in Nemo while a window is focused. (There is an outstanding bug report for Nemo, whereby if you do this on the Desktop, the resulting bookmark doesn't work properly.)