All the lands comprising in the cantonment area are to be recorded under general land register or GLR of each cantonment area. First, you take a certified copy of your survey number from the commandant of cantonment area, GLR is a public document and commandant is bound to give a certified copy.
If your land is mentioned in its any entry then it will become clear that your great grandfather was not the owner of this land. Government of India is the sole owner of every land mentioned in General Land Register, In British period these land were given on special grant that is called an old grant. At that time Lord Governor was the owner of those properties.
After independence, this right is vested in the President of India through the ministry of defence. You cannot sell this land. But you may get compensation to the extent of the cost of building, which is decided by the committee of arbitration.
In Chief executive officer vs Surendra Kumar Vakil (1999) 3 SCC, held by the supreme court that property granted by “old Grant” shall not be sold by the grantee.