There is no correct order to play the games. Each contain their own independent story (except Honkai: Star Rail which is a sequel to Honkai Impact 3rd set in the rest of the universe away from Earth). While the games do share a connected universe and many concepts which build upon each other to give a bigger picture and make things more clear, each story stands up on their own and can be played in any order. Guns Girl Z has been receiving updates alongside Honkai Impact 3rd since HI3 released and the story is still going. Zombiegal Kawai/Houkai Gakuen 1 was Hoyo's original take on the genre, though due to a variety of factors they chose to make it into a gacha game and release it as GGZ.
Currently Operation Gekkou is approaching the end of GGZ part 1 (Retrospective). GGZ currently is on its 3rd part so we still have a large amount to do and the amount keeps growing.
If you would like my opionin on where to start, I would say to either start with HI3 or GGZ. Since HI3 has the more complete story available in multiple languages that is probably the best option.
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thx for the answer i thought GGZ was finished due to the servers shutting down
thank you for your opinion on where to start i heard there was a manga or something for both i was going to look that up and see if its the entire story and read that but now I just start with honkai impact and start your project alongside it
thanks again for the answers