I followed the instructions in this article to get the correct date/time: Add data to Google Sheets with workflows (hubspot.com).
You can also convert dates from milliseconds in Google Sheets by adding the following formula to a separate column in the spreadsheet: =(A1)/1000/60/60/24 + DATE(1970,1,1). Replace A1 with the column that the unix timestamp is in.
If you need to adjust for GMT, you can add or subtract hours based on your time zone. To do this, you need to adjust the formula by adding or subtracting milliseconds from A1. For example, if you’re in GMT-3, the formula would look like this:
(A1-10800000)/1000/60/60/24 + DATE(1970,1,1).
After applying the formula, make sure to adjust the number format of the cell to “Date” in the Google Sheets settings.