These tutorials say that they are for making t-shirt designs with GIMP for Amazon Merch.
But actually the videos are really just how to make your t-shirt designs using GIMP. They are short and to the point, really well done.
It turns out that it can take anywhere from one month to nine months to get accepted on Amazon Merch. That is because it’s cost structure is very favorable. They can print and ship a T shirt for about ten dollars cost to you! Much better than other print on demand sites. See How long does it take to get accepted in the Amazon Merch program?. The author of this brief video (4 min) recommends some steps you can take to get accepted faster but no guarantee. Get on to Red Bubble or Teespring or others so Amazon can see you have a portfolio.
Gimp Tutorial for Merch #1 – Creating a New Image [means creating a new template]