- used the newly added game ID as reference for GAMEINFO to autoselect which game to start a mod with.

While in GZDoom this uses the IWAD name, the same approach is a lot more problematic here because of name duplications with far more incompatible content.
So this allows targeting a group of base games instead of one specific version.

source/core/gamecontrol.cpp

@@ -123,7 +123,7 @@ FMemArena dump;	// this is for memory blocks than cannot be deallocated without
 
 InputState inputState;
 int ShowStartupWindow(TArray<GrpEntry> &);
-FString GetGameFronUserFiles();
+TArray<FString> GetGameFronUserFiles();
 void InitFileSystem(TArray<GrpEntry>&);
 void I_SetWindowTitle(const char* caption);
 void S_ParseSndInfo();
@@ -643,14 +643,25 @@ static TArray<GrpEntry> SetupGame()
 
 	int groupno = -1;
 
-	// If the user has specified a file name, let's see if we know it.
-	//
-	FString game = GetGameFronUserFiles();
+	auto game = GetGameFronUserFiles();
 	if (userConfig.gamegrp.IsEmpty())
 	{
-		userConfig.gamegrp = game;
+		for (auto& str : game)
+		{
+			for (auto& grp : groups)
+			{
+				if (grp.FileInfo.gameid.CompareNoCase(str) == 0)
+				{
+					userConfig.gamegrp = grp.FileName;
+					goto foundit;
+				}
+			}
+		}
 	}
+	foundit:
 
+	// If the user has specified a file name, let's see if we know it.
+	//
 	if (userConfig.gamegrp.Len())
 	{
 		FString gamegrplower = "/" + userConfig.gamegrp.MakeLower();

source/core/gamecontrol.h

@@ -139,6 +139,7 @@ struct GrpInfo
 	FString defname;
 	FString rtsname;
 	FString gamefilter;
+	FString gameid;
 	uint32_t CRC = 0;
 	uint32_t dependencyCRC = 0;
 	size_t size = 0;

source/core/initfs.cpp

@@ -60,10 +60,10 @@ static const char* validexts[] = { "*.grp", "*.zip", "*.pk3", "*.pk4", "*.7z", "
 //
 //==========================================================================
 
-static FString ParseGameInfo(TArray<FString>& pwads, const char* fn, const char* data, int size)
+static TArray<FString> ParseGameInfo(TArray<FString>& pwads, const char* fn, const char* data, int size)
 {
 	FScanner sc;
-	FString iwad;
+	TArray<FString> bases;
 	int pos = 0;
 
 	const char* lastSlash = strrchr(fn, '/');
@@ -78,7 +78,7 @@ static FString ParseGameInfo(TArray<FString>& pwads, const char* fn, const char*
 		if (!nextKey.CompareNoCase("GAME"))
 		{
 			sc.MustGetString();
-			iwad = sc.String;
+			bases.Push(sc.String);
 		}
 		else if (!nextKey.CompareNoCase("LOAD"))
 		{
@@ -136,7 +136,7 @@ static FString ParseGameInfo(TArray<FString>& pwads, const char* fn, const char*
 			} while (sc.CheckToken(','));
 		}
 	}
-	return iwad;
+	return bases;
 }
 //==========================================================================
 //
@@ -144,7 +144,7 @@ static FString ParseGameInfo(TArray<FString>& pwads, const char* fn, const char*
 //
 //==========================================================================
 
-static FString CheckGameInfo(TArray<FString>& pwads)
+static TArray<FString> CheckGameInfo(TArray<FString>& pwads)
 {
 	// scan the list of WADs backwards to find the last one that contains a GAMEINFO lump
 	for (int i = pwads.Size() - 1; i >= 0; i--)
@@ -184,15 +184,15 @@ static FString CheckGameInfo(TArray<FString>& pwads)
 				if (FName(lmp->getName(), true) == gameinfo)
 				{
 					// Found one!
-					FString iwad = ParseGameInfo(pwads, resfile->FileName, (const char*)lmp->Lock(), lmp->LumpSize);
+					auto bases = ParseGameInfo(pwads, resfile->FileName, (const char*)lmp->Lock(), lmp->LumpSize);
 					delete resfile;
-					return iwad;
+					return bases;
 				}
 			}
 			delete resfile;
 		}
 	}
-	return "";
+	return TArray<FString>();
 }
 
 //==========================================================================
@@ -201,7 +201,7 @@ static FString CheckGameInfo(TArray<FString>& pwads)
 //
 //==========================================================================
 
-FString GetGameFronUserFiles()
+TArray<FString> GetGameFronUserFiles()
 {
 	TArray<FString> Files;