Huh? Change the gain of what? Why?
Is a firetruck responsible for maintaining proper water pressure at the fire hydrant it is hooked up to? No. That's not its job.
It's a DAW, not a preamp. If you have to change the "gain" of something in Ardour, you're already doing it wrong. If it could... all it'd be doing is allowing you to continue to do it wrong.
If you are feeding it a weak signal, fix the damn signal.
If you are feeding it an overly strong signal, fix the damn signal.
"Fix the damn signal" means adjust your outboard equipment (preamp, mixer, noise generator, or whatever) and/or your soundcard's mixer to get a proper signal happening to begin with. Don't feed your DAW a crappy signal, unless you WANT to record a crappy signal. Get it right as early in the chain as possible, because once you are in the DAW, it's too late. You're already past the AD conversion... any gain is nothing more than a little math to make it louder, which can be done at any time. It'll never make it "better".
If it is too late fix the signal, and you are dealing with pre-recorded audio that is too weak (or outboard equipment that isn't up to snuff), fix it right (permanently) with an audio editor. That way you aren't wasting CPU doing that same digital math over and over again every time you hit play.
And if you absolutely, positively can't do it the right way, and insist on changing your gain in the DAW itself... yes, you can do it with a plugin. (Not exactly hard to find... I found no less than three already installed, and I've never installed any but the most common/basic plugin packages. And that's on a plain vanilla Ubuntu machine, not a studio machine with lots of goodies on it.) In the end, doing this with a plugin is no different than having it built in. It is better, because it gets it off the damn screen for most people, who will not and should not need it 99% of the time.
Sounds like a lack of either minimal effort or minimal understanding of recording in general. Go get a Mac and use Garageband. It was made for you.
(And hey, I'm not knocking it. I do have a Macbook, and have used GB for sketching around with. It's not bad. Inability to export midi is probably the only really major drawback.)
If you wanna knock Ardour, say something like "it lacks MIDI functionality found in most other DAWs", which would be true. Until v3 comes out, at least.