Firstly, I'd change the battery.
If that doesn't solve the problem, then I'd recommend sourcing a genuine Ford remote key. In my experience, the aftermarket keys and cases aren't as well made and were a waste of money.
The immobiliser chip can be transferred from the old key to the new one.
The new key's locking/unlocking code can be programmed into the GEM as follows ...
HOW TO PROGRAM A NEW REMOTE KEY’S CODE INTO THE REMOTE CENTRAL LOCKING SYSTEM
Insert an existing key. Turn it from position 0 to position 2 and back to position 0 four times within three seconds. Remove the key.
The red warning light should flash to indicate that the remote central locking system is in "learning mode".
You now have 20 seconds to ...
Press any one of the new remote key’s buttons.
The red warning light should flash to indicate that the new remote key’s code has been learned by the remote central locking system.
Wait 20 seconds for learning mode to time out. Alternatively, insert a key and turn it to position 2.