Steam – the save files stored

Each saved game in XCOM is put into a folder inside the game directory, GAME_1, GAME_2, ... up to GAME_10. You can just copy/move the folder to your new computer where xcom is installed.

As for using dropbox, the entire XCOM game is so small that you can probably just have steam install it straight into your dropbox folder*, see this link for details: https://support.steampowered.com/kb_article.php?ref=7710-tdlc-0426

*This is assuming your steam library is small enough to fit into dropbox, other wise, ignore this suggestion. Either way, you can use it to transfer saves with no problem.

I had the same question, and here is what I found. Since the SteamPipe update, .GCF files are not used anymore by Steam. Instead, content is packed into .VPK files.

Source: Source engine games on SteamPipe

Now, to locate different files you may need to modify (in order to create a mod for example), you need to navigate to (example given for Half-Life 2, so for Garry's Mod you need to change the folder's name):

steamapps\common\Half-Life 2\hl2\resource.

Here you can find all the .RES files, which can be modified with a text editor. In my case, it was the gamemenu.res file, to modify items on the game menu.

Also, here is a Steam Support forum thread that helped me :

What happened to GCF's?

All the questions you may have are answered in the first link I gave (Steam Support).