It occurred to me as I was writing that I had no idea about why the Gorn had joined up, and I guess I'm still not really sure.
I felt the Gorn wanted to join because they felt bullied by Bacco during the Borg invasion and felt they needed strong allies to stand against the UFP.
Besides, with the Pact and UFP being major powers in the quadrant, I'm sure they feel the need to belong to one of these trade and defense pacts.
This has inspired me to quickly go over what I know about the Gorn, to see if I've truly got a grip on them myself.
The Gorn have always been territorial; indeed one of the few insights we get into them canonically is their extreme territorialism. They defend themselves brutally and with single-minded zeal if they feel their holdings are being encroached upon, but apparently their outlook is indeed mostly defensive. It seems to be rare for the Gorn to expand or invade. After the Cestus incident they spent a hundred years with mostly static borders, according to
The Gorn Crisis - "a century of peace and isolation". They were apparently so set upon this policy that the caste that
did want to go a-warring had to topple the government to do so.
In
Destiny, disputes with the Klingons are mentioned - the Empire holds systems the Gorn believe to be theirs by right. However, if I'm not misremembering, it was a case of the Klingons having taken them from the Gorn rather than the Gorn "playing Tholian" by suddenly deciding their border should now be
there when it wasn't before. Invasion and expansion are near-essential to Klingon policy; to the Gorn, far less so. What I'm gathering from this is that Gorn are not typically expansionist, but they have long memories. If something is theirs, it is
theirs. Indeed, even the Black Crests framed their warpath in terms of "retaking Cestus".
In
Vanguard: Declassified there's evidence of a mostly insular outlook - on the Gorn border worlds, at least in the 23rd century, law applies only to Gorn, and the various alien residents are left to do whatever they like to each other, being considered
lesser and beneath notice (presuming, I suppose, they don't get in the way of or, gods forbid, harm any Gorn).
On top of all that, the novels have further established the Gorn as a very old, very conservative culture with traditions going back many millennia. Whereas the Tzenkethi stratify on the basis of testing in youth, the Gorn castes are so long-established and tied to specific bloodlines that they're basically different sub-species by this point.
Putting all this together, I think the Gorn, even more than most species, are motivated by the desire to retain their stability and defend what they have - their territory, their traditions, their boundaries. Not as insular as the Kinshaya perhaps (they at least show up to conferences and such), but very much focused on what they already have (or had), the appeal of their ancient culture, and the need to brush aside or trample anything that threatens to intrude on their boundaries.
We're dealing with a very old race that hasn't made strides into the galaxy the way younger nations like the Federation or the Cardassians have; and no doubt this is the Gorns' choice. They strike me as a people set in their ways - they're a moderate in the Pact, I think, precisely because they're such a conservative people. As I see them, they're motivated by the certainty that, resting as they are in the deep, comfortable trough of millennia of tradition and a social order that's become almost an aspect of biology, they must be onto something good. The outside galaxy is not too important if it doesn't intrude, and if it does, that Gornian single-mindedness comes to the fore and the threat is eliminated. Except in the modern galaxy the Gorn, like everyone else, have been shaken out of their complacency. The egg has nestled in its nest for a long time, but everything was shaken up real good recently, and now they're acutely aware that if the egg is to remain safe they'll need a better nest, and more guards and failsafes in place. The Borg, the Federation trying to push and pull everyone into cooperating - it's sinking in that sticking your head into the sand isn't going to get you anywhere. Like Rom realizing that the Ferengi need to learn to function as part of the galactic community, not just its exploiters, or get left behind as a joke nation, I think the Gorn have realized that the only way to keep their ancient order strong for another half-million years is to be more actively involved in the galaxy, more cooperative.
Not with the Federation, though. The Federation is too much of a threat to their boundaries, I think (by which I mean, I think they think). Whenever I consider the Gorn relationship to the UFP, I think of Ambassador Zogozin's comments in
Mere Mortals. After K'mtok responds to the Breen ambassador with a sneering dismissal, Zogozin (evidently fed up with Klingon posturing) makes a pointed remark along the lines of "Why are you even here? Didn't the Federation annex your empire?". Obviously, he's trying to get under K'mtok's skin, but I see no reason to assume that he doesn't genuinely mean what he says; evidently as far as the Gorn are concerned, the Klingons are ruled from Paris. I assume the Gorn don't want to get too close to the UFP, lest they find their half-a-million-year-old hegemonic egg being carried around in some oversized mammal's mouth...and you can't trust such an active, nosy creature with the egg, can you?