I’d say yes. But, make sure you do a proper primer. Don’t skimp.
As for the 123 vs 128 oz. I’d say that’s the manufacturers way of making it look like a gallon still. It’s like Candy Bars. They’ve shrunk.

 

Duration is fine for a door. In the painting world, a gallon has never really been a gallon even when it said so. Gotta allow for colorants, shipping so lids don’t pop off, etc, and so on, blah, blah, blah.

 

Also most steel manufacturers specify latex paint for their doors. Oil base enamels don't flex any like latex and are prone to pop loose.

 

My metal doors are all painted have for 25 years. I remember one word of caution from the instructions that came with doors. No dark colors . They could absorb enough heat from direct sunlight to an affect the foam insulation in the doors. This was particularly true if you were also hang a storm door.