Which in this case would be "brake in a straight line to the outside", because the guy in the Porsche is quite clearly aggressively defending the inside and you had to go to the outside anyway.
It was a really odd move for you to make, and disadvantaged you more than the rude driver did with his block.
Define "not allowed". Sure it's a really, really rude thing to do, but GT Sport doesn't punish anyone for it - so in fact it's entirely allowed.
As I mentioned in the penalty thread, the notion that this, that and the other are racing rules set in stone needs to be abandoned. They work if everyone has agreed to them in advance and you're playing private lobbies, but in public people will behave according to whatever their own preconceived notions of racing are - and they may not be what yours are.
People are going to change lines several times on a straight, stick their nose up your inside when they have no business to do so and shut the door on you whether you're fully alongside or not. They're also going to punt you, crowd you off the track and sideswipe you to stop you passing as you reach the finish line. The game allows all of these things and only sometimes punishes them - there's no stewards issuing grid penalties and fines.
If you think someone has been excessively unfair, use the Report feature. But remember, unless they've been hit with penalties for it, how fair they have been is just a case of your opinion against theirs (or whoever deals with the report).