G'day,
I've flown quite a few aircraft fitted with Garmin 100s and haven't really had a problem yet with glare. A lot of this time was in outback Australia, so not exactly gloomy conditions there - much like Africa I'd imagine!
Sometimes when flying with the sun behind you it got a little difficult - just a quick hand over the display to block out the sun usually worked fine! After all, its not like you're fixated on the GPS all the time anyway (at least I hope not
)
Perhaps try changing the brightness, by pushing the ON switch a few times once the unit is on. Not sure how this goes during the day but probably won't make a huge difference.
Cheers,
TL