Using GCM with a long term fixed key is dangerous; if you repeat the nonce, you can leak the authentication piece.

Instead, what I would suggest is that you use [AES-SIV][1] instead.  It doesn't have the problems with repeating a nonce.  It is more complex, and slower, however it is far better suited if you have to use a long term key.  

[1]: https://www.rfc-editor.org/rfc/rfc5297