Using to be going with lose
“We must fight,” the Spartan* chief told his small army of brave men. They
were at a great disadvantage. There were only three hundred of them. The Persian
military had hundreds of thousands of men.
They were going to lose unless they could secure a small entrance.

Does it mean they were intented to hold out to the end of battle untill they could defense that small entrance?

Question

Using to be going with lose
“We must fight,” the Spartan* chief told his small army of brave men. They 
were at a great disadvantage. There were only three hundred of them. The Persian 
military had hundreds of thousands of men.
They were going to lose unless they could secure a small entrance.

Does it mean they were intented to hold out to the end of battle untill they could defense that small entrance?

Answer

No.
It means:
They needed to secure the small entrance.

If they did not (secure the entrance), they would lose the battle.