Time-traveling lasers!
Puzzle complexity continues to ramp up all the way to the end, with a final puzzle that demands mastery over the game's most advanced mechanics, in simultaneous combination. That's one "problem" with Gateways -- it's reliant on gameplay skills that it isn't very helpful in building up. Where a smoothly-polished game design would evolve these mechanics gradually and iteratively, training the player almost by accident, Gateways isn't shy about throwing up walls and walking away. (... I guess, to be fair, there are unlockable in-game hints, but I actively avoided them in order to think through puzzles on my own. Although I wish that some hints were available for the last puzzle, which was a real bear.)
I call this a "problem," using under-emphatic quotation marks, because this sense of overwhelming challenge does lend a certain character to the game. But the real problem with Gateways is that, outside of the puzzle designs, the platforming gameplay that ties puzzles together just isn't very well-implemented. As testing as it was to understand and mentally solve a puzzle, I had even more difficulty just wrangling the game into executing my solution, without falling off of inopportune ledges and totally screwing up strict timing and/or positioning. It's just so frustrating having to re-attempt a puzzle over and over, not because the solution was no good, but because of an awkward landing after a jump.
As for the game's other qualifications, the graphics and soundtrack are adequate but mediocre; there's no story except a total throwaway ending; and the game is technologically a mixed bag, with impressive portal rendering but a raft of seemingly-random infirmities (like what happened when I tried to change the resolution, which was, uh, nothing). Really, Gateways could be summed up as: great puzzles, mediocre everything else. But for players who like creative puzzles, it's worth tolerating the rest of the game for them.
Better than: Magrunner: Dark Pulse
Not as good as: Portal, Braid
Seriously, there's something sadistic: about posting the solution to the final puzzle in Hard mode, which is unrelated to the same puzzle in Normal mode.
Progress: 100% on Normal.