Will this Allantoin 10mg + Allium Cepa 100mg + Heparin 50IU Scar Gel work as a substitute for another brand with the same composition?

When two products share the same active ingredients here, Allantoin, Allium Cepa, and Heparin at similar strengths, they're generally expected to work in a similar way for scar care. Scar gels do need patient, regular use over weeks to months before you see visible change, regardless of which one you pick. I'd still suggest confirming with your doctor or pharmacist that this particular formulation is suitable for your specific scar before switching.