You can't specify the AP for a Repeater to use as a Gateway so far as I'm aware. The Meraki Mesh uses a proprietary routing algorithm to determine the best path to use based on a number of metrics, so theoretically it will always make the best choice (but that's theory and not practice). There's a little bit on this topic in this document, https://documentation.meraki.com/MR/WiFi_Basics_and_Best_Practices/Wireless_Mesh_Networking