The dream wasteland in its initial stage was vulnerable against infiltration if anyone who knew dream-related arts caught wind of it. The "Gatekeeper" power was without a doubt a much-needed protection.
And just like Dream Gate, Gatekeeper also came with other interesting features. For example, Angor could ask the Gatekeeper to allow people affected by his nightmare power to enter only. In this case, those who didn't meet this condition could not get in unless they found the edge of the wasteland and broke inside by using raw strength.
Similarly, he could "define" many more criteria for entry, such as women only, souls only, or plants only…
There were exceptions. After some trials, he noticed that items summoned into the dream wasteland by the Dream Whelk could bypass Gatekeeper. He could still determine their landing locations, but he could not prevent them from getting in once the whelk worked.