first off you can disable this by right clicking on GC3 in your steam library selecting properties then updates at the bottom is a check box for steam cloud

according to steam 

"The Steam Cloud stores information so your product experience is consistent across computers.

now im not sure if this is something that was done by steam or by SD but i think its transferring your save data so that you can continue to play this game from any computer you use and have steam running on

considering i only use one PC i find its useless

The big thing I noticed and demonstrated by uninstalling GC3 and removing the My Games/Galciv3 folder is that when syncing the cloud loads all of your saved games and when you reinstall GC3 it will download all of those games to your new Galciv3 folder.   Maybe useful later but right now it's a bother, because save games generally don't translate well to new releases.

So I turned off the cloud.

Uploading saved games to the cloud could very well be what I am seeing. Unless you have a very fast web connection the time it takes can get annoyingly long. My upload connect speed is only about 800 Mbps.  I am also turning it off, as I don't see saving games as sufficiently critical with the slow connect speed I have.

Many thanks for the explanation.