I'm going to tell my gf a lie.
What kind of lie are you going to tell her?
I'll say that I was the victim of arson when I break up with her
It can be hard to tell the truth but lies hurt a relationship, even when it is over. Your girlfriend and others will have a hard time trusting you in the future when they inevitably learn the truth.
Wouldn't you lie to me if you wanted to break up and didn't know how? It's the easiest way to handle this.
I would hope that I wouldn't lie to you, either, if I was in a similar situation.  If you want to break up with your girlfriend, you are free to do so, but you should be honest about how you feel.  She will find out eventually that your lie about arson was not true, and that will have consequences later on.