In that case, I don't see the need for SSGN at all, in India's case. Why would Indian Navy need the capability to attack so many land targets?
Payloads over platforms.
If you have a VLU that can launch Nirbhays against land targets, it can also be used to launch BrahMos NGs against ships. Plus Nirbhays with SAR seekers (much like Block-4 Tomahawk) can attack moving targets as well i.e. ships.
Maybe to bomb the heck out of Karachi and bypass any missile defences Pakis might have if we try to do the same from land based launchers from Gujarat?
And even if they did, attacking Karachi can be achieved by warships too, right?
It just goes back to the survivability of the force. A missile battery sitting in Gujarat is not immune to pre-emptive or counterattack by Pakistani forces. A submarine, relatively speaking, is.
But do Pakistanis have missile defences of significant enough capability to necessitate an Indian SSGN?
You don't build these things based on how threat perceptions are now, you build them based on assessment of future - we're talking about 2030s and 2040s here. The threat of Chinese amphibious invasion of A&N will be very real by that time. Any effort to retake the islands will be moot without massed cruise missile strikes on entrenched Chinese positions.
Plus we could have an increasing role in Middle East. Being a Top 3 economy and with declining US the responsibility will at least partly be on our shoulders to defend allies like Israel & increasingly KSA & UAE from Iranian & Turkish aggression, either directly or via proxies.
Look at the role played by Russian sub-launched Kalibrs in Syrian conflict. ISIS doesn't have ASW assets so a diesel sub was sufficient - but against likes of Turkey?
Pakistani coastal infrastructure (Karachi, Gwadar Chinese naval presence, etc.) will only be a small part of the puzzle.
Another question: If the SSGN are meant to be stealthily deployed off the coast of the enemy, what happens the minute that SSGN fires a volley? Won't the enemy pounce on the general area of that submarine with every available ASW asset? Its not like the SSGN can just scoot after the shoot can it?
Why not? A nuclear-powered sub is just the right kind of platform to shoot & scoot. Unless the enemy has assets directly on top of the sub at that time, there's no way an enemy asset reaching the supposed spot even 10 mins later has a hope in hell of finding the SSGN which has dived deep & moved away at a constant speed.
You can't do that with any other platform. Not with ships, not with diesel subs.