Some reviews tell that Gear of Wars' gameplay gets really fast repetitive. Is this true?

Moreover, are there any T-tiles in the game or is every "dungeon" strictly linear?

Randomized setups and certainly not repetitive. Quite a few boardgames out there that has everyone going 'wauw' are far more repetitive.
Not to mention this game will probably get expanded upon.

Thank you for your response!

However, a randomized setup doesn't preserve gameplay from getting repetitive. If the mechanics of a game lead to always the same cycles, gameplay will soon get homogeneous, no matter how the map is created. 

Some opinions about the game mention that the enemies behave rather clumsy and indicate that there is few variety in gameplay. Are these reviews  justified?

Only just got the game today so this opinion may change, but there are some obvious similarities between GoW and Castle Ravenloft/Wrath of Ashardalon boardgames.  Sure the themes differ but they are both basically co-op dungeon crawls with AI cards that control the monsters (although the ones in GoW are a bit more imaginative).  Now there are those that find the new D&D boardgames boring and repetitive, but I'm not one of those people (they present a brilliant starting set of tools to build your own games upon).  So, while I reserve the right to change my mind after getting several plays of GoW under my belt, I would hazard a guess that if you don't like CR/WoA then the same could be true with GoW.

EVERY game is repetitive to some degree.  Every game has a set of rules, and turn phases that never change.  So yes, the steps will be repetitive.  What changes is the board, the ai deck, and the Locust (enemies) in each scenario.  Not to mention that your strategy is constantly evolving and changing based on what your teammates do and what the ai does.

I find most of the reviews for this game a little unfair.  Yes there is repetition, but look at Catan, one of the most beloved and widely known games of recent memory.  It relies on steps that never change, but each step affects what your next action will be.  Same thing with the "Reliance on luck of the dice" in GoW.  It's unfairly looked at because nearly EVERY game relies on luck to some degree.  I once again point you to Catan where you can have the 2 BEST starting positions with the highest percentage numbers and still lose to someone on 11's and 12's who seems to be getting ALL the rolls.

So in a nutshell, the steps get repetitive, but because it's a fast paced game you have to be on your toes and manage your hand of cards which governs your actions and health, as well as keep an eye on your opponents and constant communication with your buddies.  It is a good game, and a fairly tough game which adds to the tension, fun, and replayability. 

Repetition is not a problem and a good thing in these games. It allows new players to get into the game faster. Repetition in game-play is not the same as repetition in strategy. 

The game does a good job at putting the players into scenarios where the players must carefully rethink their strategies. If you apply the same strategy turn after turn,  then yes, the game is repetitive but I don't see anyone winning a lot by playing this way. 

Besides,  playing with other players,  at every turn,  someone will have an opinion on what the next approach should be.  How can the game ever get repetitive that way. If your games get repetitive,  swap out the people you play with :) 

I think it doesn't get repetitive and here's the main reason why: You have a ton of choices in this game on the vast majority of turns. What I mean is that it's not one of those games where every turn there is obviously a best move to make. With those types of games, it's almost like you don't really have as many options as you think. With GoW, though, you have really, really think about your options. Should I retreat and recuperate? Should I run past everyone? Should I attack or wait and defend with an attack? I'm not even scratching the surface.