I have purchased and installed the Forgotten Gods DLC but it does not show in the DLC list at the start of the game (Like Ashes of Malmouth and Crucible). Is it integrated with the main campaign or else how do I access it? The DLC installation went smoothly with no error message and "check file integrity" on. Any help will be appreciated.

Well... It is part of the main campaign. I started a new game and after the first mission someone appeared outside the gates and when I accepted the mission he proposed it started the Forgotten Gods DLC. Thanks to MedeaFleecestealer and Wrathclaw77 for their response.

Sounds strange because as far as I know you shouldn't see Ashes in a list either. The only thing that differentiates the versions is the background of the main character screen. You should see a woman holding a skull and dagger if Ashes is installed and a guy with a sword if Forgotten Gods is installed. Only Crucible should show separately since it's a separate add on and not part of the main campaign like the expansions are.

Can you post a screenshot of your main character screen?

@ Medea Fleecestealer

I think you get it wrong,what he does mean. It sounds,like he means the GOG pulldown-menu,right next to the start/play button (called settings? / I use the german version,where it is called Einstellungen),where you can alter your start settings,the used DLCs,GOG overlay,BETA channels and so on.
And THERE,ALL DLCs should be and are listed (in my case)
Crucible,Forgotten Gods and Ashes of Malmouth,each with a checkbox!
(assuming you use GOG Galaxy 1.2,I have no experience with GOG 2.0)

@procyoniv

If they AREN'T listed in there,you should consider a new install of the DLC,because it sounds like an install error/GOG registration malfunction,had that too in other cases.Or reinstall the whole damn thing,then the program can use its own order of installation,sometimes the games suck at registering,when you install components not in "their" preferred order.

OR,if you mean the INGAME selection of game-type,main campaign or crucible (exclude custom game in our case) are the only selections and the DLC content is added to the main campaign,like in nearly every other game. (some quest guy opens a portal for you,go over there to get to the add-on,etc. etc. blabla)

a tip: start a new game and check the achievement window,if there are Forgotten Gods and/or Ashes of Malmouth content to click on,you are on the safe side and can start and enjoy the game,everything is there and should be running.

Hope you get it running! :)