I'm going to sound like a techno virgin here but I'm not sure I know what gapless playback is.... is that when you have an entire album in a single FLAC file? Because I have those and they work on the T51, but not flawlessly, sometimes you can't press next and back, so you just have to drag the scrollbar to find the song you want, or fast-forward or rewind...
The only issue with the T51 is that you have to navigate in a folder structure, it's not a DAP where you just put your music on it and then navigate by artist or genre, at least it seems glitchy there? I don't actually know, because I always navigate by folder structure, and I find it quick and intuitive, never had any issues.
One "issue" I have is battery life, which is less than ideal at around 8 to 10 hours, but the battery itself is huge, it seems like the chips and output power and large screen just don't make it a power friendly device, you can get through one full length movie.
At the end of the day, it's NOT intended to be a UI friendly DAP, it's intended to be an audiophile DAP with two DAC's, a great sounding line-out, and a great sounding headphone-out, what's fantastic about it is the price, and you get a huge screen to watch movies on as a bonus feature, is the way I look at it.