You'll need to download your Google Sheet first in order to import the contact data into HubSpot.
The import tool in HubSpot supports all .csv, .xlsx, and .xls file types. If you click on File > Download in your Google Sheet, you'll be able to select one of the supported file types.
Once you have your file, you can go to your HubSpot account > Contacts > Contacts > Import and follow the steps there. After the contact data has been successfully imported, you'll see the new contacts in your contacts overview in HubSpot.
Thank you so much for your answer, it was really helpful! Nevertheless, I'm still trying to manage another problem, regarding sending an email to a whole list, and there seems to be a problem... Is there any possability that you could help through a short zoom meeting?