They haven’t mentioned which ATGM, the fact that you’re assuming they mean SAMHO is pure conjecture. They’ve just implied it should be able to do so from the gun tube. There’s no way to say for sure.
When releasing an RFI, you have to keep things generic. The moment you start putting specific specs in it, there would be legal consequences. So you can't go on and state that the tank should have this gun or should be able to fire that missile. Now in the RFI Fire Power segment, the gun launched missile has not been kept as optional. It has been mentioned that it should be able to fire ATGM along with smart munitions.
Now coming to project Zorawar. Its a DRDO project with L&T being the production partner. Now L&T could go for 105 mm gun and turret. But since DRDO is involved in it, the chances of that is next to zero. But some other contenders like Mahindra, Kalyani or TATA could opt for it. If L&T opts for it, then LAHAT is an option for ATGM. But for them it would be financially unsound as they would have to bring in its ToT.
When we do talk about, 125 mm turret, DRDO has developed the gun and OFB was making the turret. But again chances of DRDO/L&T going with it is next to zero. Why? Because couple of days back CEO L&T mentioned that Zorawar project is complete in-house design. So involvement of erstwhile OFB is omitted here just like Hanwa. Moreover 125mm would bring along weight penalty. 125mm APFSDS round is 2 kg heavier then 120mm APFSDS round. So when weight is paramount, anybody would opt for 120mm over 125mm.
But yes, all three have their fair chances when it would come to price bid. But on technical cum financial ground, chances of 120mm >125mm > 105mm.