Monsters follow all the same rules for advancement that player characters do, unless something specifically states otherwise. This means that yes, they do get all the same class bonuses, increased attributes, and so forth that player characters get.
A potentially counter-intuitive consequence of this is that monsters can't spend their 100 PD on abilities from the chapter on creating monsters unless they have gnosis of at least 25, because that's the rule for player characters. Instead, they're limited to buying the abilities and bonuses that any character can buy when they level up. This means that, for example, a creature with gnosis 10 can't buy the Gift, because even they they have enough gnosis to have the Gift, they don't have enough Gnosis to spontaneously acquire it without supernatural intervention.
Of course, there's nothing to stop you from just building a more powerful version of an existing creature from the ground up, and calling it a "greater whatever-it-is" or some such name. But if your aim is to describe a monster that has improved its skills the same way a player character does, following the same rules that player characters do is correct.