OK - message from the local Gaia Project apologist.
First off, Gaia Project is a fantastic solo experience. It’s widely considered one of the best automas on the market and is made by the automa factory (an outsourced solo-mode contractor, heavily used by Stonemeier) who coined the term. The automa is a fully simulated opponent which is both smooth to run and convincing as an opponent. The solo mode IS available on BGA for trial.
Second, I love the game - but you have to know what it is. As for why I say it’s better than Terra Mystica, I found TM to be a hodgepodge of disparate parts. The cult tracks are over on the side, and you have to do them but you don’t really get much for doing them. The currencies are all separate. And I found the game was hard to remember (as the mechanical pieces didn’t connect) and often shut you out.
Furthermore, the different factions were imbalanced and same-y. There are 3-4 with minor variations on “easier to terraform”, 3-4 with minor variations on “further reach”, and one other category I don’t even remember now.
Gaia Project just fixed everything. In reverse order - the factions are balanced, unique, and really creative. Each one feels different and really demands a different approach, and they interact with all parts of the game. Secondly, the game is a bit more complex but at the same time more intuitive. The Cult Tracks, previously an unwanted appendix, are now tech tracks through which every other part of the game interacts. The game is less of a shuriken with bits sticking out in different directions and more of a spiderweb with everything leading back into itself.
Yes, the space is more spread out so there is no pathway blocking, but just wait until someone colonizes that planet you wanted… the fight for real estate is (ahem) real.
The arguments people have for Terra Mystical tend to be a) wood over plastic, b) fantasy over space, c) need for 5 players, or d) prefer the different style of terrain competition. Of all of those, I’d say D is the most compelling and even agree that TM does it better - but it’s not enough to overcome everything else that GP does better.
So that’s my essay on the difference between the two. As for what they ARE… they are math.
You’ve got several currencies, everything has a price, and then you have a shopping list of what you want to do and in what order. The start of each round (especially solo) is staring at the board and doing the math of how much of each currency you need, what you can and can’t do, making a trade-off and recalculating, staring at the board to try to find a way to convert that one currency into a different one, and then finally executing. It feels like A Feast for Odin in this way, a lot of planning followed by simple execution with a few re-evaluations when someone edges in on your territory.
If you like Paladins of the West Kingdom, Everdell, or Lost Ruins of Arnak, you’re going to get that writ large. Resource conversion for action chaining, trying to squeeze just a bit more life out of this round before you have to pass and move on.
But in the end, you’re doing a lot of math and algorithmical problem solving to achieve your ends.