26" has pretty much been phased out. The only bikes that really still run 26" are dirt jump bikes. That said, there are still a few rim sets for sale in 26" flavour. Worth checking Chain reaction to see what they have, and Superstar Components do reasonable wheel builds too. Halo are awesome too, and i think still do a 26" at a good price. Might be slight overkill for what you need though.
Dont know much (well, anything) about a Cube GTC, or how old, but i assume it's disc brakes so you want a disc specific wheel set, and your rear hub will need to match your groupset, ie Sram (XD) or Shimano, and how many speeds. For your front wheel, you wont have boost, so it'll be 100mm wide. And for each wheel you need determine if it's a quick release or thru axle, and if the latter, is it 12mm or 15mm.