Having spent hours looking for an answer on how to access the Gmail API with the use of a service account and saw that I can't, unless I'm using a GSuite account that it's provided with domain-wide authorization, I came here to ask you if there's a way to actually create labels using the said API and a private account. I'm using Visual Studio 2019 and C#. In the "developers.google.com" there's a tool called "Try this API" and I can create a label using my OAuth 2.0 just fine, and the .NET Quickstart found

If you followed the quick start then you probably only included GmailService.Scope.GmailReadonly. You will need to change the scope and request authorization of the user again. Note that the tutorial you are following is not for service account authencation but rather for Oauth2 authentication.

service account

string ApplicationName = "Gmail API .NET Quickstart";
            const string serviceAccount = "[email protected]";

            var certificate = new X509Certificate2(@"c:\xxxxx-api-ed4859a67674.p12", "notasecret", X509KeyStorageFlags.Exportable);

            var gsuiteUser = "[email protected]";

            var serviceAccountCredentialInitializer = new ServiceAccountCredential.Initializer(serviceAccount)
            {
                User = gsuiteUser,
                Scopes = new[] { GmailService.Scope.GmailSend, GmailService.Scope.GmailLabels }

            }.FromCertificate(certificate);

            var credential = new ServiceAccountCredential(serviceAccountCredentialInitializer);
            if (!credential.RequestAccessTokenAsync(CancellationToken.None).Result)
                throw new InvalidOperationException("Access token failed.");

            var service = new GmailService(new BaseClientService.Initializer()
            {
                HttpClientInitializer = credential,
                ApplicationName = ApplicationName,
            });