For reviewer: as I suppose few have a NetBSD system available, I wrote a guide to setup a VM for testing purposes.
This attempts to update the NetBSD docs so one can successfully build on the latest release. It also adds instructions to build the GUI.
Additionally, it includes a note and an example on how one could update the gcc version bundled with NetBSD 9.2 and prior to be able to actually compile. This note can be updated with the release of NetBSD 10, as it will package an acceptable gcc version.
Related to #20610, but reworked.