The Fake Z Fighters are illusions that appeared inside Buu's brain. Goku was forced to fight against the illusion Gohan, while Vegeta faced off against the illusion Gotenks and the illusion Piccolo. When some cake grabbed Super Buu's attention, it caused the fakes to be turned into sweets.
It is stated in Daizenshuu 7 that their power is on par with their real counterparts. This does not seem to be true however, as Super Saiyan Goku was able to defeat the fake Ultimate Gohan and Super Saiyan Vegeta was able to defeat the Super Saiyan 3 Gotenks and Super Namek Piccolo illusions.
Techniques and special abilities
The Fake Z Fighters use the same abilities as their real counterparts. However they are also able to regenerate due to being mere illusions.