The GFM, as defined in 52.245-1 Government Property,.is designed to allow the contractor to incorporate government owned raw materials into a final contract.  If the contract is set up with this in mind, then it becomes an accounting issue.  If the contract says the government will provide all the raw titanium and contractor uses 10,000 pounds of government furnished titanium at $30 per pound to make an end item, then it becomes an accounting issue.  The cost of $300,000 gets included into the cost of the end item.  If the contract was set up that way, then there is no modification required.  It sounds like what is happening is you are traying to add GFM to a contract that did not contemplate GFM.  In that case, consideration via a modification is the process.