They are like ghouls in some ways but are a different creature, and are not always friendly to ghouls. When Randolph Carter and his ghoul allies spot ghasts in the Underworld, they hide from them, hoping not to get attacked. 

On the other hand I’d agree that ghasts are probably on some level related. Both are vaguely humanoid, and both have hoof-like feet, somewhat less so for the ghouls. They have some human features too, so my guess is both species developed from degenerate humans – in the case of ghouls probably from humans practicing black magic, and in the case of ghasts it’s less clear. The description of the human food animals from The Rats In The Walls sounds almost like they are trending a little bit towards ghast-level, and that was only about a thousand years of breeding. 

My feeling is that any kind of underground animal is likely to speciate and form into variations, since it’s really easy for them to have isolated populations for thousands of years at a time. In the same way that there are probably thousands or millions of byakhee species, there are probably several dozen types of ghast. Plus the Ancients explicitly bred them for domestication so in that case they might be as different from the wild ghasts as a mastiff is different from a wild jackal.