I doubt you understand, what you are talking about.
The ONLY legal Pakistani claim to Kashmir comes from UN resolutions.
If Pakistan makes move to declare GB a part of Pakistan...UN resolution on Kashmir under Chapter Six(non mandatory enforceability) , becomes defunct as all the parties to dispute have refused to abide by the resolution.
There by taking away Pakistani claim to rest of the Kashmir.
However if the status quo remains, and war breaks out some time in the future..then India is within in its f right to capture any part of Kashmir...including the area through which CPEC passes. ..afterall Cease fire line or LOC is not IB..who ever holds the ground, owns it..and isnt that what Pakistanis tried to do in Kargil..tried to capture peaks on Indian side of LOC..in a bid to alter the LOC.
Here in lies the Chinese dilemma..can they risk investing $46 Billion in a route that passes through disputed region?
Hence recently they have pushing Pakistan to settle LOC as IB with India and make GB a legal part of Pakistan.