Can´t really compare it to G´n`G because I hate that series with a passion
(Aside from, you know, these game belonging to completely different genres). I´d compare it to the hardest Cave shmups - as a beginner you won´t last more than a few moments, but it´s a blast to play and you´ll learn very fast.
The TGM system is completely fair - the blocks have a short timeframe of "sliding" time before they lock down that resets each time they drop down, and you can even rotate the pieces in a direction BEFORE they appear - if you press a rotation button right after the last piece locked in place you´ll hear a high-pitched mechnical sound that indicates initial rotation. This can be very important since you´ll only be able to slide pieces downwards, and rotating them before they appear can avoid getting them stuck.
Of course, you´ll have to know exactly what you´re doing - where the pieces drop, how they react to the pieces around them etc. You´ll also soon discover that it´s best to arrange the pieces in a pyramid shape, so that pieces can slide from the middle to the sides. In the case of the long brick these problems are the most obvious.
Again, here is a good summary of the game basics: (In the middle part with the moving gifs)
http://bsixcentdouze.free.fr/tc/tgm-en/tgm.html
I suck at TGM too, meaning I can barely survive for more than a minute in Death Mode, a little above that in 20G and about half of that in Shirase (All in the "narrow playfield" 4 width mode I really love right now). So right now I´m aiming for simply beating my previous miniscule records, but I feel that I´m getting used to this system very quickly. (I don´t even bother looking up records or anything, I don´t do that with shmups either; beating my own scores is just a lot more encouraging.)
On my best runs I got into a kind of zone where I simultaneously watched the playing field and the Next piece window (so that my view blurred). You will really have to keep an eye on the coming pieces at all time, seing what comes next is crucial in this game. The bright colors help to identify the pieces faster after a while. You´ll also notice the sounds that announce the next piece - each one has its own distinctive melody (though I didn´t bother with learning these yet).
I always thought that Tetris had a structure that can be learned very fast and instinctively. I played Tetris Attack/Puzzle League as well as Lumines, but somehow Tetris still has the least concious thinking involved, it just really clicks with the brain somehow. Now this game combines the Zen zoning out with the hectic split-second reaction times that can be found in shmups and fighting games, to a point where it doesn´t feel like the "old" Tetris at all anymore. At first it´s not apparent how much these streamlined elements change the experience, but I can only recommend to anybody to try it for a while until you discard it.
This is the DDP:DOJ of puzzle games, very hardcore, very sweet!
These modes are obviously targeted towards a hardcore minority, but it has the same punch to it as danmaku shmups. Thanks for making Tetris badass, Arika 
