This is a generic error message that means that their servers weren't able to respond to your request in a timely manner.
Here are the typical reasons for this:
- QuickBooks Online is experiencing at outage. You'll have to try again later.
- Your request takes a long time to complete. QuickBooks sends your request from the request-processing server to a data-processing server. If the request-processing server hasn't heard back from the data-processing server within a certain time, then you'll see this Gateway Time-out message, even though the data-processing server is still processing your request! In this case, you'll just have to wait it out, check back in a few minutes to see if your request did go through. If not, then try again. Be sure to wait a sufficient amount of time before retrying. If your request is to add a new sales receipt, you don't want to have duplicates.