Razzi's ability reads: When a friendly ship declares an attack, you may spend a TL you have on the defender to reduce its agility by 1 for that attack.

Say I have Bossk(crew) and gunner on her and I miss my first attack with her. I believe they both occur in the same timing window so I can choose which one to use first, however could I use the TL from Bossk to trigger her ability then do my second attack with gunner?

There's no limitation on when the target lock can be used, so you should be able to use it for the gunner attack.

Yes, this works.

The second attack triggered by gunner does involve declaring an attack, so you should be able to trigger Latts Razzi at that point.