Authorization - Create and unlock a User
Some quick remarks:


Don't name things "class", e.g. HelperClass.
Don't use Hungarian notation and give your variables meaningful names: u_gvid is a complete mystery to me.
I don't get why CheckIfUserExists and CheckIfUnblocked are public, and even less why they'd use RedirectToAction. I expect these methods to return a boolean, not to execute redirects etc.
NpgsqlConnection and NpgsqlCommand don't follow the Microsoft naming guidelines: they are compound words, so the "s" in "sql" should be capitalized.
Don't write ADO.NET code. You're using 15+ lines to write something that could be expressed much simpler by using Dapper. The var result = cmd.ExecuteScalar().ToString().ToLower(); and if (result == "false") bit is even worse. You don't provide us with the relevant SQL, so perhaps there is a good reason to return a boolean-like string, but I wouldn't be surprised if the query logic could be much simplified.
CheckIfUserExists and CheckIfUnblocked are IMHO bad method names. To me they should be something like DoesUserExist and IsUserUnblocked, which are the kind of methods I'd expect to return a boolean.
CheckIfUserExists should not execute CreateUser, unless you rename the method. And I wouldn't expect a method called CreateUser to return a boolean. Avoid this temptation by moving these methods to a service etc. instead of them being part of the controller. Keep your controllers as light as possible, things like data processing etc. should be handled by dedicated classes.
In CheckIfUnblocked you have a boolean: var result = (bool)cmd.ExecuteScalar();. Why then not simply return this instead of doing the whole if (result) return true; / else return false; dance? You spend six lines on something that could be expressed by a single one.