@hurairaidrees Yes it will damage mehran sync. type transmission
The GL-5 oil is not ideal for sync. based as it has high-pressure (EP) additives with S and P i.e. sulphur and phosphorus.
Mehran’s gearbox uses brass/bronze synchronizers and GL-5 can chemically attack them.
You may not feel now but with passage of time, you’ll get difficulty engaging gears, especially 2nd and 3rd. 3rd gear will not come during driving due to non matching of synchronizer speeds and 2nd will be engaged but with hard difficulty and with loud grind noise.
You can drain it now and flush it so that all residue and remainings get clear from transmission then pour Gl-4 type oil in viscosity SAE 90 or 80w90 or 75w90.
Do not waste oil, Collect drained oil of GL-5 and flush oil in different containers, filter them and use them in your house old door hinges and old bicycle chains 
️ for smooth flow.
In winter 5-6 years ago, my transmission caused too much noise on cold start so mechanic added 75w90 to cater that noise issue and that vanishes noises
