The Gray Glacier network upgrade changes the parameters of the Ice Age/Difficulty Bomb, pushing it back by 700,000 blocks, or roughly 100 days. This has also been done in the Byzantium, Constantinople, Muir Glacier, London and Arrow Glacier network upgrades. No other changes are introduced as part of Gray Glacier.
With Ropsten now transitioned to proof-of-stake, the difficulty bomb only affects the Ethereum mainnet. This means Gray Glacier will not be deployed on any testnet.