Gym was boring. If he was going to run around and do things he'd rather it be constructive.. like.. working out the thumbs in a good eight hour video game session. Clearly much more effective than seeing how many crunches you can do in a time span of five minutes.
As to why Mao was in detention? Let it be known that when Gulliver's mun first read that, he laughed super hard IRL. Gulliver, also laughed, figuring there were plenty of places in math class to imply a jokingly sexist remark... then again, there always were places to do that. Not that Gully was sexist, he's got his own feminine ways, but its all for the humor of life. He knew very well that females were actually better at processing calculations than males were... so says biology and aptitude tests. When the teacher 'grred' at them in her shushing, Gully shrugged his shoulders and pointed to Mao. He was clearly the perpetrator in this.
Grinning, Gulliver shook his head. "I didn't do nothing to you, Mao," he replied. There definitely was some proud guilt on his face, but he was a firm believer of 'innocent until proven guilty'. Nothing was thrown, at least not from him. "Maybe it was that kid over there?" He questioned, pointing to some brute that was across the way from the two boys.
"Oh, hey, what're you doing after school?" There ya go, give'm the ol' 'switch the conversation' technique!