I don't know how it works in your country or how your girlfriend is, but I think you would be able to tell her without ending in a hospital. Don't tell her that you have a date or that you have already the tools (SN or a gun or whatever), but tell her that you are depressed and maybe that you have suicidal thoughts. If she loves you, she deserves to know and you deserve her support. She may freak out, but it will be way worse if you CTB and she didn't know about it.
But seriously, my "friends", my coworkers and my therapists/psychiatrist all knew about my suicidal thoughts and all that happened was that my psychiatrist told me once that I could enter a hospital if I wanted. Unless it's a really extreme case, I wouldn't worry for that.