Gunner card reads:

I', not an English native speaker, thus my doubt. If my ship is stressed I cannot use Bossk?

Correct. If your ship is stressed, you can not use Bossk.

And if your ship is not stressed, you must use Bossk.

Do you have to use him if you are unstressed but have already made a bonus attack?

In what context could this occur?

Slaver with Bossk and Cluster Missiles for example.

10 minutes ago, BoskiChe said:

Slaver with Bossk and Cluster Missiles for example.

Bossk triggers off a primary attack, which Cluster Missiles is not.

To the best of my knowledge there is no way to be attacking with a primary attack, having already used a bonus attack (or currently using one), and have bossk equipped, as the only ways to make bonus primary attacks are other Gunners, and Scum has nothing with double gunner slots.

So fortunately, it's a moot point.