If gabriel can be killed with a angel blade why cant lucifer i know this is a old question

Lucifer probably could have been killed using an angel or archangel blade, but no one thought of using one against him.

it could be that he was an angel but he have become a demon so he might be a dangel now and an angel blade won't kill him

He's pretty damn fast and strong. Gabriel was an archangel and still couldn't beat him, maybe Micheal could though. Having the blade doesn't mean you can get a hit in. It wouldn't be as easy as with Zachariah.

Normal angel blades don't work on Archangels so they have special blades (Archangel Blades) that can kill other angels but are the only thing that can kill each other. Lucifer cannot be killed with one of the normal angel blades since he is an Archangel and he used his Archangel blade to kill Gabriel. (don't worry I thought about this a lot when I first watched Supernatural)

Gabriel wasn't killed with an Angel Blade he was killed with his own Archangel Blade which is more powerful and rare to come by since only Archangels possess them.

I think it's because certain fallen angels can't be killed , like Lucifer.