Is there a way to change/edit the build steps for gated checkins on Visual Studio Team Services/Visual Studio Online? Currently it mirrors the same steps we have set up for our development branch (ei publishing drop files when it doesn't really need to) and looking at the build definition where we've got gated checkins enabled there doesn't seem to be any options which specify such a thing.
No, there isn’t such feature on Visual Studio Team Services.
Just clone a build definition and remove/disable steps/task, then check Gated Check-in for this build definition.