Not very different than what India did by militarily occupying and then annexing Hyderabad and Junagadh.
I believe Indian forces entered Junagadh in 1947, though it hardly matters - J7K, Junagadh and Hyderabad were not related. If you believe Pakistan to have been wrong in J&K, you should not have done the same yourself.
Given that Junagad/Hyderabad gambit was used by Jinnah and Liaquat Ali Khan at the UN, and yet couldn't make any headway, it should give a hint of the argument's inherent flaw.
Considering that Kashmir shared boundaries with both the countries, India was indeed an interested party all along. Military action by Pakistan, in the garb of tribal infiltration, even before the ruler had decided on the matter of accession, was therefore an act of unilateralism. This was contrary to Junagad/Hyderabad situation.
Secondly, Operation Gibraltar was the result of Indian delaying tactics and eventual unilateral annexation of J&K in violation of her commitment to the UNSC resolutions - it was in dispute territory.
Firstly, Kashmir was (is) a disputed territory which as per UN mandate is required to be administered by the respective countries that held the territories. Secondly, if India's annexation of Kashmir was violation of UNSC resolutions, so was Pakistan's annexation of Northern Areas and gift of Kashmir to China. Thirdly, by launching Operation Gibraltar, Pakistan first violated the cease fire agreement, which required standstill at the LoC, and then, violated India's right of administration of Kashmir, pending plebiscite.
If Pakistan is justified in conducting 'Operation Gibraltar', it only confirms that Pakistan is a hypocritical, unreliable, irresponsible, irredentist state, which would not hesitate to resort to military solution if something doesn't suite its fancy.
India's support for terrorists in East Pakistan was the destabilization of sovereign undisputed territory - there is a huge difference.
And supporting Naga and Mizo terrorism circa 66-70, via East Pakistan, was not 'destabilization of sovereign undisputed territory'.
That aside, stoking terrorism in Kashmir is a blatant violation of UN writ, the very writ you swear by every now and then.