Western celebrities champion causes, I doubt many of them know everything about the issues they are advocating. For example, if a celebrity says they support education for girls in developing countries, many won't know the statistics, or have spoken directly with girls from those countries, etc... But they support the cause because the little they do know about it, in theory is a good cause.
Similarly, Gambhir was not the only Indian cricketer that replied to Afridi, Dhawan and so many others did. I highly doubt most of them know much about the conflict, other than how their great army is eradicating terrorism. Do they know the root causes about the freedom struggle there? Do they know the statistics of the people that want freedom from India? Do they know all the human rights abuses that the army conducts?
Also, to be fair, Gambhir would only know what Indian media reports. Biased propaganda is hardly knowing about a subject.
But yes, Afridi should be a PHD in the subject to voice his opinion. Because Pakistanis want someone fully versed in English, be a scholar on the topic to voice any concern.