I have the DCT and the walking forward/reverse, so I can't help you with the trouble, but I did a search in the 2018 Gold Wing Forum section and found 3 different threads that may help you.
Fred Harmon does not yet have a video on the manual shift reverse adjustment.
Adjustment is under the left side radiator shroud using the in-line adjusters found under the left side deflector panel lid, from page 6-17 in the Honda manual.
But this entails removing all the plastic panels and the shelter to get to the adjusters.
You can check for cable play by removing the small black plastic panel on right side, and you can take out most of the slack by adding a zip tie as a temporary fix.
I just had mine done at the dealership where I purchased the bike. They stated that Honda wanted to charge me for the fix because it was an "Adjustment" not a maintenance issue. I argued with the dealer and they ended up picking up the cost so there was no charge but something is wrong when they claim it is not a maintenance issue even though the bike would not go into reverse.