get the car thoroughly checked it could be something other than head gasket, but a simple check will be to open the oil filling cap and check if there is sludge of oil+coolant. A normal car without any gasket issues should have a clean cap without any white/brown residue.
Secondly, drain the cooling system including heater core, and use a good radiator flush i.e: stp or liqui moly, Costs 750Rs and run the car for 10-30 KM, or keep it in for a few days on 3000-3500rpm with heater on full so it will also clean heater core, drain it and fill with distilled water only. Drive with Distilled water for one week and check if the water changes color.
There could also be a very minor head-gasket leak , which is your sign to retire the old engine and get a new one. Engine overhauls are not very good for your model of city and they donot tend to last as much as the new engine did i.e: 300k