They removed even more of Gadget's brain in order to make rooms for more gadgets.That explains why he's even less competent despite having received upgrades.
The Gadget family members each suffered a breakdown between the original Inspector Gadget and Gadget and the Gadgetinis.The stress from constantly trying to keep her uncle from accidentally killing himself finally got to Penny, and one day she just couldn't do it anymore. She stopped helping him on his missions for a while, and instead started reading up on robotics, mostly to distract herself from worrying about not knowing what was going on with her uncle or if he would come back alive. Brain continued to try to help Gadget for a while, but even during the run of the original show he was suffering pretty badly from babysitting Gadget, and without Penny's help he just couldn't do it. He took even more abuse than usual without Penny there to guide him, and eventually he started to hate Gadget. He told Penny that he couldn't stand to live with Gadget anymore. Chief Quimby knew the whole time that the girl and the dog were the ones solving the crimes, so when he found out he arranged for Brain to be retired somewhere Gadget would never find him, and made more equipment available to Penny so she could build some sort of replacement for Brain. Meanwhile Gadget continued on his missions. He had always sort of known that he had been helped by them but he could never admit it to himself, his ego wouldn't let him, he had been such a good detective before the accident. Through his trademark sheer dumb luck he had somehow managed to pull through previous missions without the help of Penny or Brain, but he wondered why his niece had withdrawn and his dog had disappeared. Penny told him Brain had been hit by a car, and used that as an excuse for why she was being distant and why Brain was gone. Gadget, in a rare moment of insight, noticed that she never took an interest in his missions anymore, never asked to go along, and was spending a lot of time alone reading, so he figured she must be mad at him, and came to the conclusion that he must have been the one who hit Brain with a car. You can bet he felt pretty bad about that. So with Brain gone and Penny spending a lot of time away from him, he was alone at home so he started throwing himself into his missions (Capeman had moved back in with his mom when she moved away to California, and now worked as a computer programmer in L.A.) He struggled to do things right and keep oblivious to the fact that he was useless, and tried so hard that his own dumb luck couldn't keep up with him. One day he went on a mission and didn't return. Penny had been home completely alone for almost a week when she got a call from Chief Quimby. Gadget had fallen into a MAD trap and been severely hurt. Most of his gadgets were broken, and they had to go in and retrieve him. He was in pretty bad condition. That's when she started on the gadgetinis. She built two of them, because she remembered her parents and how they had died and she couldn't do anything about it, and she made them to look like her uncle, because she wasn't sure if he would live, and she wanted to remember him. After Gadget's condition had stabilized Chief Quimby decided it was time to relocate them, so pulled some strings in order to get him made a lieutenant. Penny continued to build the gadgetinis while Gadget recovered and received upgraded gadgets, and Chief Quimby kept them under the radar from Dr. Claw during this time. Finally the gadgetinis were completed and Gadget was fixed, and started on regular missions again.
This Gadget is actually a robot double of the Gadget from the original series.He's much more naive, and almost ADD at times. Then there's all the additional gadgets he has that the original didn't, many of them coming out of places they never would have in the original, and the fact that he needed to have his brain "rebooted" in one episode. Additionally, he is much more childlike in this series, once interrupting a dance with a beautiful woman to play a game of keep-away.