1) both using the provided fastboot3ds and building my own from the latest source, I CANNOT launch GW from fastboot3ds. Black screen every time. I can boot using luma->bootctr9->gateway.firm.
2) If you update past the latest firmware supported by *hax ropbin, you can no longer use the homebrew launcher under GW. Why hasn't a direct launcher been made specifically for b9s and/or fastboot3ds? heck, I'd even take a rosalina (under/for GW) launcher at this point.
interesting, because the fast boot method initially worked for me, using fastboot3ds to boot the gateway.firm directly, then one day i got the same issue as you, just a black screen, using fastboot and bootctr9 works, and thats what i use now
the gateway.firm is the gateway a9lh payload, that was converted to a .firm, this allows you to boot the launcher.dat
iirc gateway launcher is encrypted, so one has to first figure out how to decrypt and then disassemble that code and then one could simply reverse engineer it, then one can have native 11.14 support with out frakenand