This may be unrelated, but perhaps worth a mention... after I updated my system I could go through the login part of the GW Admin Console, but then the little wheel would just spin. I had to clear all browser cache, cookies, etc. It then worked without a problem. Perhaps you try the same and see if it makes a difference?
Did you apply HP2 directly to a GW 2014 system? Or were you already on GW 2014 R2? The reason I ask is that R2 changes the certificates a bit. That said, if I understand correctly, your browser is simply complaining that it doesn't recognize the certificate, correct? If so, I think all you need to do is "accept" the certificate in the browser. You know it's trusted as a self-signed certificate.
So, you are saying your "logon" fails? When you say "trust", I thought you were referring to the certificates. But perhaps we just have a language miscommunication here. If you are saying you cannot login because the user/password does not seem correct, are you logging in as the gwadmin user? Or were you given rights as a system admin with your own userid?
We can reset the GW user/password from the command line if that is your issue.