Hey, hope I can help you. If you haven’t done this, this is what you do. when you click export (on the file tab at the top or ctrl + shift + e) you need to click on Select File Type (by extension) at the bottom. Then you scroll down and click on PNG. Once you have done that just name it what you want and click export. Every time you export something after that it will be as a PNG unless you change it.
Hope this is what you’re after, if you have any other questions about GIMP I’d be happy to help. 