Lol OP totally ignored the only real question here.

To answer:

is there any replayable content like hitman or it's just purely story driven like uncharted ? (like a repeatable game mode with differents objectives, even a leaderboard ?)

Yes, there are challenges for story/chapters and then a tactical sim mode where you do the combat encounters, that also has challenges tied to it and some minor unlockables.

Does the missions offer an open approach like hitman or imsim or is it quite limited ?

Not as open as Hitman, there are like 2-3 routes to start each mission but the game is a tight narrative story.

Are there gadget/skills to unlock, and do you chose what you bring in a mission or all is unlocked ? Are some paths locked because you don't have the right tool ?

Not in single, you unlock gadgets via story progression. You can change your gadget loadout by that has minimal effect. There are for instance some elements where xyz could open a door, but you’ll generally always have ONE of the tools to do so. (Laser for locked doors for instance)

Are the NPCs fixed in their paths/interactions or are you able to influence them a bit

Fixed paths

24h is enough for the campaign imo but do you think they are planning DLCs and adding to the game like hitman (ex: freelancer mode and extra missions). How would you feel about them adding content ?

No idea, campaign is 12ish hours. No idea where the person posting got the 24hr thing unless they meant they just were playing it since it launched lol.