What you can do is have a three way connection like this:
Google Sheets -> FTP -> Automation
I'll be leaving the connecting the Google Sheets to an FTP part for you to Google as I haven't done that before as well, but with a quick search I saw that it is possible and it can be automated.
If you set up an automation in SFMC, with its Starting Source as a File Drop, that should do the trick for you.
When you use the File Drop option, the automation will run when the defined file (with its defined name) is present in the FTP. The first and only step you should use, based on the information you provided above, is an Import File step. In that, you can select the csv file (or w/e format you're using with which delimeters) within the FTP, then select which Data Extension you want the file to be uploaded to.
Based on the information you provided in your question, I'm assuming you'll be using this quite often and you don't want to modify the "Import File" definition every time a briefing is submitted to you. So you can use this Data Extension as your Master File, and have all the assets provided to you uploaded in that same Data Extension. If that is the case, I'd suggest to include a unique name for each briefing, and include that unique name to every row for every briefing, so you can use lookups to get the assets you need.
The method I described above requires the stakeholders (people who are submitting the briefing to you) to fill in the Google Sheet with a defined format and correctly. Otherwise, you'll have to modify the briefing before exporting it to your FTP.