What Is This Shit: Gus tells his family to go and do one, so he can run off with Scott instead of marrying Torie.
Gus had never thought of himself as particularly brave. He definitely wasn't the type to stick up for ideals, especially when doing so put him at risk. Really, it was much better to keep his head down, tell people what they wanted to hear, and then do what he wanted on the sly. Safest, easiest--just the best way, really.
Until it wasn't. At first, when he'd gotten together with Scott, it didn't seem like anything had to change. Yeah, they went from messing around casually to being exclusive, but Gus was still marrying Torie when he graduated, was still going along with his family's wishes, and it didn't even occur to him that Scott could make him want something different.
It had crept up on him, really. At first, it was just little things, fondnesses and bits of trepidation about the future that could be easily pushed aside or explained away. But then his parents started the wedding planning in earnest during Christmas hols, and Augustus realised that he wasn't just displeased with having to marry Torie--he genuinely didn't want to marry anyone other than Scott, and he wasn't going to be content with breaking things off at graduation, or carrying on as an affair.
A few months of moping around and mulling the issue over, and Gus finally came to the difficult decision that he had to be honest with his parents. If he didn't, he was going to be properly miserable for the foreseeable future, and being miserable would probably give him unsightly lines.
He hadn't told Scott about his decision, either. He knew that Scott would tell him not to do it, would martyr himself for Augustus' happiness, and really that would only make things worse. Best not to upset him further until things were already settled, was what Gus had decided, and so he said nothing on the train, just kissed Scott goodbye and acted as though that was the end.
As difficult as that was, waiting for the right moment to have the confrontation with his parents was possibly even harder. It took a couple of weeks, until finally Gus told them that he wasn't going to be marrying anyone other than his boyfriend. Predictably, they reacted quite poorly once they realised he wasn't going to be so easily persuaded this time, and when they offered him an ultimatum (get married or get out), they were quite surprised at the option he took (get out). Gus had left his things packed from school, had just added to them with what he wanted from home, and so not five minutes after that argument was over, he was heading out the door, hopping the Knight Bus to Scott's house. Sure, he could have apparated, but in his current state, he didn't think it very wise.
After a couple of hours of the bus driving up and down the countryside, it arrived at his stop and Gus descended, lugging his things with him. He probably should have been more worried about how Scott was going to react, but at the moment he was mostly focusing on holding himself together. Once he reached the McDermott doorstep, Augustus pasted a smile on his face and reached out to knock it, twice.