Masters are as oblivious as everyone else about everything else about their Servants to begin with (unless they used a catalyst to know exactly who they'd be getting), then they just naturally learn more about their Servant as they see them in action or the Servant tells them things, or if the Master gets random dreams about their Servants pasts.
That is, that's how it normally is. Of course, those lines can be bent for a roleplay, but since Gummy is saying he'd post Servant CSes with only the bare minimum, that would normally suffice to begin with unless otherwise desired. I don't think he can actually stop you from sending random PMs, but yeah. Haha.
