Initially I had no problems with using my C-Gear on my Pokemon Black 2 cartridge however when I used my game on my DS lite and then attempted to play the game on my 3DS I'm no longer able to use the C-Gear without getting a connection error.
I honestly believe that whatever emulation that is used to simulate the DS (original) wifi and connection software had became corrupted and is the source of the problem.
All the internet connectivty for my 3DS works fine for other games and the store however whenever I attempt to enable C-gear in game it blue screens my game an dstates that there was a communicatoin error and it needs to be restarted.
I upgraded my DS to the most recent version and then formated it down to factory settings with no success of resolving this issue. I firmly believe the resolutoin of this issue is beyond my control and request a further support from nintendo regarding the matter.
Once connected, the Nintendo Wifi Connection treats the system/game as a set. I assume changing systems caused the problem, since the game's wifi settings were tied to the old system. The following was taken from page 15 of the Pokemon Black Version 2 manual, hopefully it helps you.
"If you change the combination of your Nintendo DS system and Pokémon Black Version 2, you need to do the setup again to connect. The registration on the Pal Pad (see page 16), and your Rating (see page 18) will also be deleted unless you do the following. To transfer your settings to
another Nintendo DS system, here's what to do. For the Nintendo DS or Nintendo DS Lite, got to
the "Options" menu in the "NINTENDO WFC SETTINGS" menu. For the Nintendo DSi or Nintendo
DSi XL, go to the "Options" menu in the "Internet" menu in "System Settings." Then, select