Will GMod ever be sold in stores?

I don't think so, sorry.

Do they ship it to me or do I download it?

Steam will download it automatically for you, so you'll be able to play almost as soon as you buy it!

What games do I need to play GMod?

You need at least one Valve Source Engine game, there's a list of them here. Steam won't let you buy Garry's Mod on its own if you're unable to play it.

How do I set up a dedicated server?

See the wiki article Dedicated Server Setup

If I only have CS:S/TF2/Portal/DoD:S will I still be able to use the HL2 props and ragdolls?

Yes!

GMod Freezes or crashes! Help!

The answer to this is almost always to upgrade your video card's drivers. Steam Support has a useful page on that, see Updating Drivers. If you're using an integrated Intel chip, you might want to consider upgrading. See this page for details. Finally, if you've installed addons or mods they might be causing the problems. See this page for Cleaning GMod]

Do we pay once, or every month/year/update?

Just once.

Will I get CS:S ragdolls and props and maps?

If you own CS:S you will. If you don't you won't. Same with DoD and Half-Life:Source. If you only own CS:S you will get everything from Half-Life 2 apart from the maps and the speech.

Will I get TF2 ragdolls, maps and props?

Only if you own TF2 on the same account you own GMod on

Will I get Portal ragdolls, maps and props?

Only if you own Portal on the same account you own GMod on

I just installed GMod 9 and...

Stop! Sorry but GMod 9.0 is no longer supported by me. The answer to all GMod9 questions is "Get GMod10".