Can I replace rear drum brake of Gixxer SF with disc brake?
Yes, the rear drum brakes can be converted to a disc brake but it is complex task and such modification will also be expensive, but why is your emphasis on rear wheel disc-brake? The reason of not providing rear disc brake by the company is that when we apply the brake, the front wheel brake has to do more amount of work as compared to the rear wheel brake therefore disc brake is provided on front wheel as it is more efficient than drum brake, the rear brake is not as crucial as the front, also replacing the rear drum brake with disc won't help you stop any faster as Gixxer SF is not a super sport bike.
For replacing drum brake with disc brake you would require a major tear-down of the wheel, entire wheel. tire, valve stem, wheel weights, ring gear, and especially and the drum brake assembly needed to be disassemble, you will need adapter plate to drill mounting holes and studs and bolts etc will be needed for the installation of disc brake. We don’t advise you for such modification but still if you have crave for rear disc brake then contact some experienced mechanic as he will do this task for you more precisely.
Questions about Suzuki Gixxer