Can I share some Google stuff without sharing Gmail?
Any music you have purchased can be downloaded as MP3 files. Once downloaded, it can be uploaded to the Google Music service in another account. My wife and I do this with CDs we've purchased and ripped to MP3. IANAL, but since you are speaking of your wife, your property is shared and your music is owned by both of you.

This, however, does not work for the "All Access" features Google Music. If I understand the terms correctly, that account cannot be shared.