Are Mandolins easy to restring?

Yes, restringing a mandolin is a relatively straightforward affair. Just do as you would with a guitar, keeping in mind that the pairs of strings are all tuned in unison, NOT in octaves like a 12 string guitar!

What notes is a Mandolin typically tuned to?

Taking it for granted that each pair of mandolin strings is tuned in unison (not octaves), the normal tuning for mandolins is GDAE. We sometimes refer to this as 'backwards bass' since the notes are the reverse of how you'd tune a bass guitar.

On a mandolin, what gauge is regarded as 'light' gauge?

Everybody has different opinions on this sort of thing but it is generally understood that, on a mandolin, 'light' strings refer to gauge 10 strings, normally 10-34.