What year RT? My 2009 had a sunlight glare problem. Is your GPS screen contrast/display set to max? I don’t know if this is a BMW factory GPS mount or an aftermarket unit but some people have installed bezels to knockout some of the glare. 

Here is an earlier post with an example. 

HI sharkbait,

I have a garmin zumo mounted to a single ram ball handlebar mount, on my 2016 R12RTLC.

Are you needing one of these to fight off glare?

Sun Shade Glare Visor shield Garmin RV 660LMT 770 Drive Smart 60LM 60LMT 70LMT | eBay

The one I got was about 5 years ago when I had my 2010 12RT.   Works fabulous, never looked back.

It required some slight modification due to the barrel nature of the side walls of the zumo but, that was remedied with a heat gun.

Mounts with velcro, or 3m tape.

Good Luck.

Thanks, however I found an exact fit visor for my Garmin from this link:  

https://www.google.com/aclk?sa=L&ai=DChcSEwjB0Pj3wY_vAhX4KK0GHVzMDYIYABBUGgJwdg&sig=AOD64_1ZzExlF6P3F_klhp6hs5--YD1zmw&ctype=5&q=&ved=0ahUKEwiK8_L3wY_vAhXCLH0KHZjbCAEQpysIQA&adurl=

Since my Garmin mount had all the adjustments as part of the base mount (left side) I was able to use this portable shade to the Garmin and when removed from mount the visor stays with the Garmin for storage.