Treaty of Accession is an unproven document, nor does India possess the original document if any ever existed. If you want to argue Treaty of Accession, then India should have honored the demands of the Muslim Emirate in Hyderabad Deccan region.
You have no legal claim, no original document, no original signatures, etc. Besides, Gilgit-Baltistan and it's people are an integral part of Pakistan, and unlike what happens in IoK people are demonstrating or calling for liberation and self-determination, these people actually fought on Pakistan's side in 1947.