The real question I am trying to answer is what would have transpired without the gatekeeper getting a message.
The reason is, the only apparent thing the gatekeeper does accomplish is getting Harry to use LC before it is ready. Harry would still get the phone call, and would still be involved without the note, right? What did the gatekeeper actually change?
The Gatekeeper has ways of knowing things, but he doesn’t seem to me to
see all and know all. He may not have been aware of the possibility that Molly would call Harry. Or didn’t think it was that likely.
I see the Gatekeeper as working with, or at a minimum doing a favor for Mab in PG. Molly I think is going to be nearly as important as Harry when the really big turds hit the fan when we approach the BAT. Harry becoming a nickel head and Molly losing her head by way of warden sword dose Mab and the team she sometimes works with no good what so ever.
At the time of PG Harry is likely the only person who Molly the rebel teenager would take direction from, and Harry is clearly the only wizard who would stick his neck out to help her. Likewise, I think the taking on of Molly as a troubled apprentice helped Harry reaffirm his beliefs in the what’s and whys of magic. This reaffirmation likely helped Harry resist Lashel’s shadow’s temptations.
Harry and Molly were both each other’s moral anchors.
Also I agree with neruro's assessment that the Gatekeeper knows the fine line Harry is walking and is ready to kill him if he goes dark, but is also willing to help him as he can as long as he doesn’t go dark.