No, they're not aliens. The first ones were formed from the "godstuff" that every planet has in its formative years, which is why planets throughout the universe all have gods. The rest are just the biological, godly children of those gods. Hell, Thor's mom is Mother Nature, so he can't really get more tied to Earth than he is.
There is indeed a biblical God in the DC universe. Zauriel, Asmodel, and all the other angels, arranged in four hosts that match the animals associated with each of the gospels--eagle, bull, lion, and man--all report to Him. Eclipso was His divine wrath back in the Old Testament days and the Spectre is His wrath now.
The DC universe also has a bona fide Satan--the biblical Lucifer, for whom there's been a whole Vertigo series, which is in continuity because it's from back in the days when DC still allowed Vertigo titles to publish stories about mainstream DC characters. He's even said in the Sandman series to be second only to God in power. Of course, Mike Carey takes a decidedly Milton-esque approach to Lucifer, presenting him with some heroic qualities, and he and Gaiman threw in a bunch of weird quirks to top DC's Lucifer off--like the fact that he got bored of ruling Hell and abandoned it, leaving a couple of angels to rule it in his stead.
Personally, I love the addition of Judeo-Christian religious figures in DC's cosmology. I don't really draw any distinction between the figures of those religions and the ancient myths' gods, except that the former are clearly intended to be far more powerful and a bunch of people still believe in them. Plus, avoiding them just makes everything seem far faker and more manufactured. It's ironic that Marvel only just recently added a true Satan to its ranks, rather than its running commentary that various demons, including Marduk Kurios and Mephisto, have posed as Satan but none are the true being, since Marvel has always prided itself on being the "more realistic" of the big two universes.