There seems to be a good number of people with these problems. In the last couple of weeks, I have noticed that my Wii U Gamepad no longer has any range. Right now, I'm sitting on my couch, not 6 ft from the Wii U console and the connection is first "unstable" and then "lost" and the Gamepad turns off.
The console updated today, and I resynced the Gamepad just 30 minutes ago, but the problems are not fixed.
I have had the Wii U for almost a year now - I bought it April of 2013 - and just this past March I was playing virtual console games on the couch with the Gamepad all the time. I used to take it out on the balcony or into another room and play without any problem...and now I can't even get it 4 ft away from the console.
Most of the games I play don't require the Gamepad, so that's a relief, but I still need a solution. This just suddenly happened, and the Gamepad shouldn't be breaking after less than a year without any mistreatment - it just sits on the cradle most of the time.