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Dont reveal until either the start of the GSC first movement phase or the end of my movement phase so until the start of GSC first movement phase they cant be shoot at? Shouldnt it be a "whatever comes first" in there? Havent actually seen the text so im only quoting someone else.

If the gsc go first it's at start the gsc movement phase. If the non gsc player goes first it's after the non gsc movement phase.
So they will ways be revealed by the first shooting phase.

Whoever you are quoting is making a mistake.

If the GSC player has the first turn, they reveal ambush blips at the start of their movement phase.

If the opponent (you in this case) has the first turn, the GSC player reveals ambush blips at the end of the opponent's movement phase.

Therefore, provided you aren't doing any shooting in the movement phase, you will be able to shoot the GSC units after they are revealed.

If you go first, you do movement phase. Once done. They place all their models that need to be placed with blips. You then enter psychic phase, then shooting phase where you can shoot at them, then assault then end turn. You go second, they have to reveal, movement, psychic, shooting, assault, so on your turn yet again you can shoot. There is never a time they can’t be shot at unless lack of LOS or out of range