Is a blown gasket a warranty claim with my 2013 Holden Cruze

The gasket in question is probably the head gasket and when this fails, it can allow the engine oil and coolant to mix, which is why you’re seeing oil in the coolant expansion tank. If you check the engine’s oil dipstick, you may find that coolant has found its way into the sump, mixing with the oil as well, giving the latter a milky quality rather than appearing clear and clean.

This would have been a potential warranty claim back when the car was new, but at 11 years of age – even with your low mileage – it would be a miracle if Holden agreed to fix it as part of a warranty job.