He's determined, sure, but he doesn't exactly come across as particularly courageous or chivalrous. I know families tend to all fall into the same house, but of all the Weasleys we meet, Percy definitely seems the least Gryffindoresque. So why?
As discussed before on this site, the Sorting Hat doesn't sort based on specific event(s) but on a person's potential (which is why it Gryffindored people like Neville "Timid" Longbottom, Hermione "Nerd" Granger and Peter "The traitor" Pettigrew).
As such, the fact that Percy joined the Battle of Hogwarts in the end shows that, as usual, the Hat wasn't wrong.
Percy: "Hello, Minister! Did I mention I’m resigning?"
Considering that he fought Augustus Rookwood and later Pius Thicknesse (who was a former head of DMLE and therefore a formidable opponent), he more than proved his bravery.
While he was a blithering idiot judgement wise, it did take courage to defy his family and stand with the Ministry. Would YOU risk the combined wrath of Molly Weasley, F&G twins, Charlie, Bill AND Ginny?
He's courteous (which is listed as one of the traits of Gryffindor) and, presumably, chivalrous to a degree though the latter isn't really explored in canon much.
Apologies about the lack of sources and links - I'll go back through and add them once I get home and can access the books/sites.
While I think @DVK's answer is good, I'd like to offer an alternate opinion.
Percy was sorted into Gryffindor because he chose to be in Gryffindor. Both of his parents and his older brothers were in Gryffindor, and - presumably - probably several generations before that. So he wanted to be there too. There are several instances where the Sorting Hat takes the candidates choice into consideration.
The most obvious one is Harry himself, with his "Not Slytherin!". It's also referenced that Hermione could easily have been in Ravenclaw, but due to her reading about the houses she preferred Gryffindor.
Neville Longbottom is an interesting counter-example, because I believe I read in Pottermore that he asked to not be in Hufflepuff, because he wasn't brave enough to be in Gryffindor. But his reasoning wasn't sound - he showed his bravery throughout the books in many ways, large and small, and the Sorting Hat obviously could see his potential.
Percy's driving feature is his sense of right and wrong. His personal ambition (which might have sorted him into Slytherin), his critical thinking (which might have sorted him into Ravenclaw) and his sense of fair play and peacemaking (which might have sorted him into Hufflepuff) were all secondary elements to this core personality trait, and in many cases took a back seat to it.
That's not to say that he doesn't embody those things that would have fit him into the other houses, he clearly does in many respects. The one we see the most evidence of is his ambition at the Ministry, but there's little evidence he would have started down that path if he had realized how wrong a position it would have put him in - indeed, once he realized the truth of what was going on, his shame at being on the wrong side greatly overpowered his ambition.
That's Gryffindor all the way.
Very simple. Because the author decided it shall be.
J. K. Rowling already KNEW how the entire series of books would play out and that required people to be in their places long before the final act was played out.