 If you liked
Gantz
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 ...then you might like
100-man no Inochi no Ue ni Ore wa Tatteiru
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The games in both manga follow very similar rules:
* Characters are transported to an alternate world/space at some intervals
* They are given Quests which they have to complete in order to return
* Life on the line, if everyone dies the "game" is failed
There might be even more similarities ("game masters" are strange looking men who give out quests using their strange 'lingo') So if you like manga with such game elements it might be interesting for you. Gantz is mature audience oriented compared to 100-man, however both stories get rather complex and interesting as story goes on!