There currently is no official update for the N1 to Gingerbread yet.
However, you could root your phone and install CyanogenMod 7 (nightly, it hasn't hit release candidate yet) to get Gingerbread on your phone. From what I've heard, the nightlies have been pretty stable for the N1.
More details on the CM wiki here.
Basic steps include:
- Root your phone
- Install ROM Manager
- Flash ClockWork Recovery
- MAKE A NANDROID BACKUP OF YOUR PHONE
- Flash CM7
Note: rooting your phone will void your warranty. For more info about rooting, see here and here.