Is there a field in the spreadsheet file that uniquely identifies each row of data in that file? If so, it may be possible to set up an "update matching" type of import where this unique identifier is selected as a match field so that re-importing the same data matches it back up with the previously imported copy of the same data.
Yes, I do have auto-enter serial number for each record upon import. However, since my google sheet source does not have the id field, my source id get reshuffled after the import so each record ends up with a different serial id. Is there a way to 'lock' that id field so it stays with the record? Or is there a way to omit previously imported records from the new import and only import new fields?
You misunderstood my question. Is there a unique identifier of any kind in the Spreadsheet? If there is such data--even if just a name field, then my suggestion will work.
There is an update existing records option that might serve, but would need to be used with extreme care as a change in record order in either file will produce a major data mismatch on you. Better to match by an ID if you can come up with one in the spread sheet file.