var core = new DefaultGameCore
{
    ClientToken = clientToken,
    RootPath = rootPath, // Path of .minecraft/
    VersionLocator = new DefaultVersionLocator(rootPath, clientToken)
    {
        LauncherProfileParser = new DefaultLauncherProfileParser(rootPath, clientToken)
    }
};

var launchSettings = new LaunchSettings
{
    FallBackGameArguments = new GameArguments // Default game arguments for all games in .minecraft/ as the fallback of specific game launch.
    {
        GcType = GcType.G1Gc, // GC type
        JavaExecutable = javaPath, //The path of Java executable
        Resolution = new ResolutionModel // Game Window's Resolution
        {
            Height = 600, // Height
            Width = 800 // Width
        },
        MinMemory = 512, // Minimal Memory
        MaxMemory = 1024 // Maximum Memory
    },
    Version = versionId, // The version ID of the game to launch
    VersionInsulation = false // Version Isolation
    GameResourcePath = Core.RootPath, // Root path of the game resource(.minecraft/)
    GamePath = path, // Root path of the game (.minecraft/versions/)
    VersionLocator = Core.VersionLocator // Game's version locator
};

launchSettings.GameArguments = new GameArguments // (Optional) The arguments of specific game launch, the undefined settings here will be redirected to the fallback settings mentioned previously.
{
    AdvanceArguments = specificGCType , // GCType
    JavaExecutable = specificJavaExecutable, // JAVA's path
    Resolution = specificResolution, // The window's size
    MinMemory = specificMinMemory, // Minimum Memory
    MaxMemory = specificMaxMemory // Maximum Memory
};

launchSettings.Authenticator = new OfflineAuthenticator
{
    Username = "Username"
    LauncherProfileParser = Core.VersionLocator.LauncherProfileParser // launcher_profiles.json parser
},

var result = await Core.LaunchTaskAsync(launchSettings).ConfigureAwait(true); // Returns the launch result