Gildart and Hardaker set to start for GB against Tonga
But George Williams looks set to miss out - to his replacement at the Warriors, Jackson Hastings.
The Australian-born Hastings is at halfback alongside Gareth Widdop in the 21-man squad, which will be trimmed to 19 on Friday, the day before the game.
Hardaker played just one game for Wigan at centre this year, against Salford, but has been preferred to Hull FC's Jake Connor.
The 21-man squad suggests Wayne Bennett's line-up but he will be able to shuffle the ranks before kick-off if he chooses.
Kick-off on Friday is 8am UK time.
Great Britain squad: Coote (St Helens); McGillvary (Huddersfield), Hardaker (Wigan), Gildart (Wigan), Hall (Sydney Roosters); Widdop (St George Illawarra), Hastings (Salford); Hill (Warrington), Hodgson (Canberra), Thompson (St Helens), Bateman (Canberra), Whitehead (Canberra), Graham (St George Illawarra, capt). Subs: Jones (Salford), T Burgess (Souths), Lomax (St Helens), Walmsley (St Helens). Reserves: Connor (Hull FC), Hughes (Warrington), Williams (Wigan), Clark (Warrington).