NO! Do NOT use reverse until this is corrected. Probably just an adjustment of the tunbuckles under the right side cover. It should click to lock in and lock out and make a "whirring" sound when in use. No grind!
PRS is right... if it is actally "grinding" like something is getting chewed up in a blender, you have a problem.
That said, I'm wondering if you are new to GL1800s and if this if the first time you've ever heard the reverse operate on a GL1800. If so, maybe you're just surprised to hear that much "humming". And it does hum pretty loud.
Excuse me if you know this, but if not, this may help you understand the sound that reverse makes: Reverse on a GL1800 does NOT use the engine in the bike to back up (you're in neutral). So, if you are expecting to just hear the normal sound of the engine running while backing up, you are going to be surprised, and wrong as well.
Reverse actually uses the electric starter motor on the engine to make the bike move backwards. When you push in the Reverse button on the right handlebar, and then push and hold the Start button, the electric starter motor drive gear engages a special reverse gear in the transmission and drives the bike slowly backwards on battery power alone. What you are hearing is the electric starter motor humming! If it's just that humming you are hearing, then you are OK. (The starter motor does have a bit of the sound of an electric power tool when it is running, if that helps describe it.)
If you are hearing more than just the starter motor humming, like metal-on-metal grinding or something, then you have a problem.
Hope that helps. Again, sorry if you already know this.