For a game that was primarily developed by one person, Gettysburg is a seriously impressive undertaking. The variety of weapons range from period stuff, such as cavalry and ironclads, to the crazy futuristic steampunk weapons like tanks and zeppelins. You can organize battles where both sides can only use period-correct or futuristic weaponry, or a little of both. Battlefields are absolutely massive, and you can hop into any unit at any time and fight in the battle alongside your troops. Online battles have been dead for a long time, sadly, so you'll likely spend most of your time whupping the AI, which is pretty good.
Now, unfortunately, there are things wrong with Gettysburg. Graphically, it doesn't look as good as the screenshots make you think, but it doesn't look that bad either. There is tons of lag in game, which doesn't seem to have anything to do with what specs you have. Biggest complaint would be Paradox forcing the game into an early release and then having killed off the multiplayer servers quick as could be, dooming the game from the get-go.
Folks into alternate history and anything far-fetched should get a little fun out of this, but due to the online being dead, I'd say hold out for a 75% off sale before grabbing it.