whether Arjun's GUN is better than T 90?
Yes. It is. Greater accuracy , ability to fire more modern ammo, and longer range.
And whether WE can indigenously make a INDIAN GUN
Yes. We already have it in the Arjun.
which can share commonality with T 90 tanks
NO. Unless you put an autoloader and go for the same split ammo philosophy as in the Russian tank, and just like the Russian tank, if there is a penetration are prepared to see the crew go up in a pyre and the tank blow it's top off.
Remember, the British want to go the Rheinmetall way not because of performance issues, but because of ECONOMICS and if they standardize on the Rheinmetall , they can do away with the entire supply chain, ammo and upgrade problems wrt to the gun on their Challengers . Their tank force is small and limited. Small numbers go against fundamental economics.
India's is the reverse. We are the guys with the "volumes". We can be the guys who drive changes.
That said, finally, I have a question.
Fundamentally , Why should we go for a smooth bore gun ? Especially if the current gun is better than than the 125mm on the T90 . Earlier, the Indian Army "harrumphed" (esp the DGMF I think) and said you cant fire a Anti Tank Rocket from the barrel of a rifled gun.. Of course you know better now (look up YouTube videos of Arjun firing the LAHAT)..That knocked the bottom off the "oh.. Rifled guns are so backward ,they cant fire rockets, we must get smooth bore " camp of the IA. Plus there are advantages to the rifled gun like firing long range HESH munition.
Of course rifled barrels wear out faster than smooth bore and are more difficult to make. But then, if any tank like Arjun fires enough munitions to wear out rifling, it must have fired 1000s of shots in anger and fought a big serious war and survived it to lived to tell the tale, something we cant say of the T series.