Yes, you can. When the main A83 is closed, there are usually 3 alternative routes you can take:
Convoy - During daylight hours, a vehicle convoy often operates on the 'Old Military Road' adjacent to the main A83. Follow the usual route through Tarbet and Arrochar to take this route, but first check with Traffic Scotland to ensure the convoy is operating.
Ferry - There is an increased service on the Western Ferries Gourock-Hunters Quay crossing during road closures. On exiting the ferry terminal at Hunters Quay, turn right onto the A815 through Strachur & St Catherine's before taking a right on to Hells Glen (B839). Discounted ferry tickets are available from Pauls Food & Wine in Gourock.
Diversion - Turn right at Tarbet and take the A82 to Tyndrum. Join the A85 to Dalmally, then the A819 to Inveraray. Then take a left onto the A83, following through to the other side of the 'Rest'.