I ran 3 #6 B/R/W and 1 #8 Grn from my main panel through 1" sch 40 PVC to my garage and will be installing a 100A sub panel with separate ground and neutral blocks. I understand that since it is a separate building I need a grounding rod. I would like to know if I could run a # 8 bare copper wire to one of the two grounding rods of the main panel. The rod is about 7 feet from the wall I will be attaching my new sub panel to. This garage is 100 years old and solid brick only the roof is wood if that makes any difference. And should I install a 60amp GFCI breaker in the main panel for the main shutoff?
It's not a separate building
Having a breezeway between the buildings means it is one building in the eyes of NEC. You not only can use the same ground rods, you can simply use the ground wires you already ran.
Further, you do not require a shutoff switch (typically implemented as a main breaker) in the garage's panel. Although if you're committed to it, it does no harm.