I routinely tune my mandolins GCEG, aka open C, slak-key C or taropatch C. For the E and G courses (formerly A and E courses) I use .017 and .015 (non-wound) strings. The G and C courses (formerly G and D courses) are the standard mandolin wound strings.
I come from a 50-year 5-string banjo background; this tuning is like strings 1-through-4 on a 5-string banjo with a capo at the 5th fret. I also play plectrum banjo (flat picking) and square-neck Dobro (finger picking), so this is a very natural tuning for me.
This tuning tames down the high strings on a mandolin very nicely, plus it's a pretty low tension tuning for those who prefer that for their mandolins. You do lose about 9 half-steps from your top range with this tuning, in my case I play barre chords (and chord-melodies) up the neck quite a bit.
Don't be put off by this, many pros make tuning accommodations to play the instruments that they play -- for example, many types of banjos have been tuned like violins, mandolins, guitars and ukuleles for over a century for the convenience of experienced players of those instruments. There is even some historical implication that the mandolin itself took on violin tuning for the same reasons. There are also a large number of less common alternative tuniings for most stringed instruments.
In my case most band mates, jam partners, and often even experienced mandolin players, cannot tell that I'm playing my mandolins in a non-standard tuning. But, if you are used to standard mandolin tuning and you try to play one of my mandolins, don't be surprised if you feel like a mandolin newbie again.