Pratchett was legitimately responsible for like 80% of that book. It's mostly his stylistically and in terms of language. Gaiman came up with a lot of the story aspects, but Pratchett did the bulk of the actual writing because Gaiman was still doing Sandman full time and Pratchett was on a break from Discworld.
Also, Pratchett said some things over the years about Gaiman that can be...open to interpretation in terms of how he viewed him. I've also read where a lot of people have claimed that the two were nowhere near as close as Gaiman liked to portray they were. And I did always find it odd that in the 25 years between the publishing of Good Omens and Pratchett's death, they never really collaborated again despite Pratchett being an absolute machine in terms of output and collaborating with other people.