I would say: " The mission is a goer", meaning that permission has been granted for the mission to proceed.

Refer to my comment - Mission here could be Military, or a Rocket launch, or a voyage of adventure - one that has approval to get underway.

It's scarcely real English, at best awful English and probably of American origin. I guess it is a lazy way of saying: 'the status of the mission is 'GO'' - or 'the mission has a 'GO' status.

It means the mission is confirmed and it will start within a given time period. If the military of a country finds out where hostages are being held, and it's decided to attack this place to free these hostages, the order "Mission is a go" may be said to confirm the attack will take place. As for another way to say it, I think "Let's roll!" is very close.