.ga domains are allowed in Google Search Console. The failure to fetch your XML sitemap is not caused by the top level domain.
Submitting an XML sitemap to Google Search Console is usually not necessary and your site should do fine with SEO even if Google can't fetch the sitemap. XML sitemaps let Google know about all the pages on your site, but they don't help your site get indexed or ranked better. Whether or not you have submitted an XML sitemap, Googlebot will find and crawl your site. If you have linked every page from some other page, Googlebot will use those links to crawl your entire site even without an XML sitemap. If you haven't linked to every page, you should do so. Links to pages help Google decide to index pages and help rankings way more than XML sitemaps do.
To diagnose why Google can't fetch your XML sitemap you should:
- Check that you typed the sitemap URL into search console correctly.
- Copy the sitemap URL out of search console and try access it in your web browser yourself.
- Find a 3rd party sitemap testing tool and run a test.
- Use the URL inspection tool in Search Console with a "live test" of the sitemap URL to see if you can force Google to try to fetch it again.
If you find a problem with the above steps, correct it. Problems that can cause this error are:
- Putting the sitemap URL in wrong
- Server errors for the sitemap URL
- Badly formatted XML in the sitemap file
However, Google is known to show this error fairly commonly because of problems on their end. Maybe their internet connection is flaky, or maybe there is a bug in their sitemap fetching code. We don't really know. In any case, if there are no problems on your end, the error will likely go away on its own within a few weeks. It is Google's fault often enough that a hosting company, Wix, has it in their FAQ: https://support.wix.com/en/article/google-search-console-couldnt-fetch-error