Ground News isn't a source per se. They provide links to various news sources so you can see different viewpoints on the same story. They try to align the sources left and right and indicate their factuality but that is just meant to be a guide to allow you to challenge your viewpoints or determine factuality and to make it clear what the ownership structure might be guiding them to write.

I've been a paying subscriber for 4 or 5 months after finding them and find it quite reasonable and interesting.

https://ground.news/rating-system

tl;dr they buy those ratings from press rating agencies and then average them.