I have found "Galt" ON through Mapquest, but it takes me to Cambridge, ON and then there is no other reference to Galt on the map. Is the city/town of Galt no longer?

Galt still exists but is now part of the city of Cambridge. See Ontario Locator for a synopsis of the amalgamation.

I am searching for my g-g-aunt and think I found her in the 1871 census listed at Rockwood Lunatic Asylum. Is there anyplace I can write to find out if it is her and any info on her?

If this is a provincial hospital, the records are held in the Ontario Archives on Grenville St., Toronto. If it is a private facility, then I believe the records were not required to be kept much beyond 20 years, if that.