Which is a huge problem for all of us :p
Seriously though, the convenience is basically the same as GoG (actually, even more convenient, but whatever). You can either dig out your cd-r, or double-click something.

Uh... which... if you scroll up... was kinda EXACTLY what I was saying...
To put too fine of a point on it, though, you can install GOG's version of something if you're offline, as long as you've downloaded it. I *think* Impulse needs you to be online for the install, not just downloading. I obviously haven't tried this...
Anyway, who cares, right? Total Annihilation! Woohoo!

I have to say I'm annoyed
Last year, i saw this game for $4 in a CD rev store, bu I had vowed to myself not to buya gam for 6 months.
Damn

I've heard of it, never played it.

Well I did keep that promise to myself, so yes, but I missed out on Total Annighilation for $4

I hope they will add all TA games later.

I've seen TA: Kingdoms at Frys Electronics still as of I guess late last year. Ten bucks if I recall correctly.

I'll be waiting for it to be available via GoG or go freeware. I'm not personally comfortable with DRM which requires you to be connected. Whether just for the install or for play that's a nonstarter for me.
I tend to keep a set of copies of things like this on my HDD, I don't usually bother to back them up, or at least not often, but I can in most cases install it without having to be online at all.

Hm, I don't know if I'm ready to play against decent people yet...

Is there even a way to play it against people yet?

Yes, Very much so. The multiplayer portion of this game was basically the sole reason there are so many maps, units and full game packages for it.
It appears your running pretty blind here so i'd suggest a quick search, this link will fill you in on the basics
http://en.wikipedia.org/wiki/Total_Annihilation
Anyway...
This game is genuinly awesome and blows its rivals out of the water with its real time resource management and the sheer scale of control that you have. It works in a very logical way with unit production for each factory being independent and it definitaly proves that men CAN multitask! I really hope this is brought on to gog because i think the community here would lap it up.
As you may guess this is one of my favourite games and I'm a bit a veteran with it, so a few tips to the budding commander...
* You have 2 types of unit, construction and military. I'm yet to find a reason why any of these should be standing still.
* Radars are your both your freind and enemy, be intelligent with radar jammers and your opponant won't know whats going on. in the same way, don;t always believe what it tells you. scout with cheap units to see if areas are actually clear or hold a massive factory base.
* And finally, build, and when you've done that build some more! regardless of the size of the map - expand your borders as much as you can!

The Boneyard may be gone, but there are currently ways to play it Multiplayer. In time Impulse should be integrating their own MP Server system, but that hasn't happened yet. You'll have to use something like GameRanger, GameSpy, or LAN. Hope that answers your question.

Is the community active at all? I played nonstop for years, but eventually the online community became so sparse that there was no point looking for a game anymore.

To be honest after playing a bit I would rather TA: Spring with XTA or BA mods. Comes with lobby, tons of players across the world, more units, better balance of units, and better graphics that run fine on even my single-core linux PC on Ubuntu 9.10.
That's just me of course, TA is still a great campaign and skirmish and awesome memories of my youth.