The name is certainly English but not much is known for sure beyond that. Some sources claim that it comes from the River Gilpin in Cumbria while others suspect that the family name came first and the river name later. This is probably right since family names seldom if ever come from river names. At any rate, the meaning of the name remains unknown as far as I can tell. You'll find the first Gilpin to come to America at
http://awt.ancestry.com/cgi-bin/igm.cgi?op=GET&db=gilpin1&id=I905