Does syncing iphone to a new computer erases notes linked to the gmail account?
The erase and sync function is only for items synced from the previous computer in regards to media.
Outlook would have been the only application on a PC that would sync your notes.
So if you haven't used outlook on your PC, then you should be fine to erase and sync. 
Also, if they are through your gmail account this will be unaffected even if they are erased, because by signing into the gmail account it will sync the notes over the air which is separate from any syncing with a computer.
Best wishes!