Will bars leaks rear main seal repair work on a bad head gasket?

No, a Bar's Leaks Rear Main Seal Repair product will not fix a bad head gasket. These products are designed to swell and seal leaks *in* the rear main seal itself, a completely different part of the engine. A head gasket sits between the engine block and the cylinder head, sealing the combustion chamber. A blown head gasket will cause issues like coolant leaks, oil contamination, and loss of compression – problems completely unrelated to the rear main seal. Using a rear main seal repair product will not address any of these issues. You need a head gasket repair or replacement to fix a blown head gasket.