GalCiv 1 only had one expansion, Altarian Prophecy. It added two new races (Drath and Korx), two campaigns, several new technologies, planetary improvement, ships, and other minor content. There might be more, but I'm not that familiar with GalCiv 1. I only got it a couple years after the sequel came out.
GalCiv 2 had two expansions:
The first one, Dark Avatar, added two more races (Korath and Krynn), a new campaign, Mega Events, a new spying system, an overhaul to the tech tree along with a couple new techs, extreme worlds (planet-types you needed to research first, before you could colonise them), asteroid mining, a couple new planetary improvements, some changes to diplomacy (along with economic and research treaties), Super Abilities (a special ability for each race, which made them play more differently), new ship components, and more. It's been a couple years since the last time I've played DA (not to mention the base game), so I don't remember all of the changes.
Twilight of the Arnor, the second expansion, had an even bigger list of changes. You can read up all of the major ones in this post
from the devs (though the screenshots no longer seem to work). TotA eventually got a major update in form of v2.0, which added a whole slew of QoL improvements. You can read up on some of them here
I hope this helps.