Essentially, what they are saying is they swapped out their 4.0 litre engine for a 4.6 litre engine. GEMS refers to the Engine Management System (Proper name is SAGEM) also known as the ECU, or, simply the computer.
From late '95 till the end of series 1 Disco, their fuel injection and Ignition timing (distributorless) was controlled by the Gems system. Prior to that Rover used a LUCAS, Hotwire system, and with the advent of the series II disco came the Bosch computer, considered by most to be the best. Personally, I like the old Lucas system, because the average Joe can work on his truck without needing a $10K computer.
The 4.6 they used in the swap came out of a 95-99 1/2 Range Rover 4.6 SE or HSE. After 99 1/2, RR also switched to Bosch.