Known issues and limitations:

GalaxySDK may be initialized only once per process, so each player window must be spawned in a separate process
Dedicated servers are not supported yet

UE4.20

Multiplayer is broken for UE4.20 due to FUniqueNetIdRepl serialization bug in Engine. You can either cherry-pick the fix done by Michael.Kirzinger@epicgames.com from UnrealEngine repo(commit d2c61c90e1c203bd89d868950b552e5af7e0fe20), or apply diff-patch with these changes(Source/ue4.20-unique-net-id-serialization-fix.patch).

Installing plugin:

Copy the plugin folder to your "GameFolder/Plugins"
Unpack content of the Galaxy SDK to "GameFolder/Plugins/OnlineSubsystemGOG/Source/ThirdParty/GalaxySDK"
Enable the plugin:
- either via UE4 Editor interface:
  - Settings -> Plugins -> Online -> OnlineSubsystemGOG
  - check "Enabled"
  - restart UnrealEditor
- or modifying *.uproject file manually as follows:

"Plugins":
[
	{
		"Name": "OnlineSubsystem",
		"Enabled": true
	},
	{
		"Name": "OnlineSubsystemGOG",
		"Enabled": true
	}
]

Update default engine configuration file ("GameFolder/Config/DefaultEngine.ini") providing values without angle brackets:

[OnlineSubsystem]
DefaultPlatformService=GOG

[OnlineSubsystemGOG]
ClientID=<CLIENT_ID>
ClientSecret=<CLIENT_SECRET>
; The local port used to communicate with GOG Galaxy Multiplayer server and other players. Chosen automatically if not specified. Can also be overriden with -port=<port>
; Port=<LOCAL_PORT>
; The local IP address the game would bind to. Chosen automatically if not specified. Can also be overriden with -multihome=<host>
; Host=<LOCAL_HOST>
; When authorizing with Galaxy, this flag controls whether user can play offline (using local profile from the GalaxyClient), or has to be logged on to Galaxy backend services
; When offline, achievements, stats and other data are stored locally until user is online, and some functionalities are unavailable, e.g. friends, multiplayer, rich presence
bRequireBackendAuthorization=<IS_BACKEND_AUTH_REQUIRED>

[/Script/Engine.Engine]
!NetDriverDefinitions=ClearArray
NetDriverDefinitions=(DefName="GameNetDriver",DriverClassName="/Script/OnlineSubsystemGOG.NetDriverGOG",DriverClassNameFallback="/Script/OnlineSubsystemUtils.IpNetDriver")

[/Script/Engine.GameEngine]
!NetDriverDefinitions=ClearArray
NetDriverDefinitions=(DefName="GameNetDriver",DriverClassName="/Script/OnlineSubsystemGOG.NetDriverGOG",DriverClassNameFallback="/Script/OnlineSubsystemUtils.IpNetDriver")

[/Script/OnlineSubsystemGOG.NetDriverGOG]
NetConnectionClassName="/Script/OnlineSubsystemGOG.NetConnectionGOG"

Please contact your GOG.com™ tech representative for more info on how to obtain ClientID and ClientSecret

XBOX cross platform support:

Make sure your engine is compiled with the support for the XBOX. Please contact Epic Games™ representatives for more details.

Make sure both Microsoft XDP and GOG.com™ backend services are configured to use XBOX authentication for Galaxy. Please contact your GOG.com™ tech representatives for more information on how to configure this.

To compile this plugin for the XBOX, you will need binaries from both Win64 and XBOX versions of the Galaxy SDK unpacked to "GameFolder/Plugins/OnlineSubsystemGOG/Source/ThirdParty/GalaxySDK"

Update XBOX engine configuration file ("GameFolder/Config/XboxOne/XboxOneEngine.ini"), adding XBOX TitleId and PrimaryServiceConfigId:

[OnlineSubsystem]
DefaultPlatformService=GOG

[/Script/XboxOnePlatformEditor.XboxOneTargetSettings]
TitleId=<XBOX TITLE ID>
PrimaryServiceConfigId=<XBOX SERVICE CONFIG ID>

Rest of the config file is similar to the default engine configuration file described above.

Authentication:

In order to use online functionalities user must be signed in GOG Galaxy Client after which game must be authorized using one of the above methods:

Using C++, IOnlineIdentity::Login() method is provided:

auto onlineIdentityInterface = Online::GetIdentityInterface(TEXT("GOG"));
auto onLoginCompleteDelegateHandle = onlineIdentityInterface->AddOnLoginCompleteDelegate_Handle(0, FOnLoginCompleteDelegate::CreateRaw(this, &OnLoginComplete));
FOnlineAccountCredentials accountCredentials;
onlineIdentityInterface->Login(0, accountCredentials);

void OnLoginComplete(int32, bool, const FUniqueNetId&, const FString&)
{
	Online::GetIdentityInterface(TEXT("GOG"))->ClearOnLoginCompleteDelegate_Handle(0, onLoginCompleteDelegateHandle);
	...
	// Proceed with online features initialization
}

Using Blueprints, you can find Login method under the Online category

Steam-based authentication

Before authenticating using Steam EncryptedAppTicket both AppID and PrivateKey have to be configured first in GOG Devportal. Please contact your GOG.com™ tech representatives for more information on how to configure them.

Since Steam EncryptedAppTicket constains binary data it has to be first converted to a hex string, then passed to the Login method along with Steam user name:

FOnlineAccountCredentials accountCredentials;
accountCredentials.Type = TEXT("steam");

uint8 rgubTicket[1024];
uint32 cubTicket;
SteamUser()->GetEncryptedAppTicket(rgubTicket, sizeof(rgubTicket), &cubTicket);
accountCredentials.Token = BytesToHex(rgubTicket, cubTicket);

accountCredentials.Id = TEXT(SteamFriends()->GetPersonaName());

Online::GetIdentityInterface(TEXT("GOG"))->Login(0, accountCredentials);

XBOX-based authentication

To authorize in Galaxy using XBOX authentication method, XboxUserId has to be passed to the Login method:

FOnlineAccountCredentials accountCredentials;
accountCredentials.Type = TEXT("xbox");
accountCredentials.Id = <XboxUserId>

Online::GetIdentityInterface(TEXT("GOG"))->Login(0, accountCredentials);

Using the Achievements:

Prior to using the achievements:

Achievements must be defined in GOG Devportal
Achievements API Keys as defined in Devportal should be provided to engine configuration file:

[OnlineSubsystemGOG]

+Achievements=ACHIEVEMENT_KEY_1
+Achievements=ACHIEVEMENT_KEY_2
... e.t.c.

bschatt/galaxy-ue4-oss-plugin

Supported baseline: