I'm trying to make a homebrew Gunblade weapon, and I want to have the actions to shoot a bullet from it or slash with the blade both under the attack actions, but j can't figure out how, could yall help me out?

My recommendation would be to break it into two entries on DDB, one sword, one gun. You'll probably save a bunch of effort that way.

If I were to do that I wouldn't need to use homebrew at all, which isn't what I'm trying to do

I don't know what to tell you, then. As far as I understand, magic items on DDB can't grant actions.