It will not be easy to reach things there behind the subpanel.
A nice option would be to use
https://advancedpanel.store/products/rv-14-map-box-ribs-pair to stack audio panel (the you already have it there, probably mounted in some other way) and transponder on top of it. In this setup, both can be easily pulled out through the panel opening once you unscrew the right GDU.
Since transponder is longer than the audio panel, back connectors on both will be accessible from below. Also, the space behind the transponder in this setup leaves sufficient space to place the ground block there on the back side of the firewall.
GDL can then be mounted on the small removable rails between the panel and subpanel on the passenger side (the space for some reason is frequently used to mount a huge transponder that honestly barely fits there) - this will ensure the proper GDL orientation.
The only stretching I had to do while installing things this way on my build was to screw transponder/audio trays to the ribs. Even riveting ribs to the firewall was a one person job.