the Access point that are repeaters arw down due the gateway Access point is down
I'm not sure but I believe a reboot of the repeater will have it pick a new gateway.
they dont , that smy problem and the main issues now is that the main gateway is showing down on dashboard.
so how to configure the new gateway or make the AP's to pick up new gateway
According to the following doc, the mesh AP will select a gateway on bootup.
I suspect in your circumstance, the gateway AP isn't reaching the dashboard but is still online. Therefore it is being chosen as the gateway.
Do you know why the gateway is showing offline in the dashboard? Can you turn it off (Eg. Disable the switch port if it's PoE) to force the repeater to find a new gateway?
You can choose in Dashboard which AP you'd prefer the repeater to use - for more info see: https://documentation.meraki.com/MR/Wi-Fi_Basics_and_Best_Practices/Wireless_Mesh_Networking#Preferr...
This is helpful, but is there a way to have a dedicated AP as a gateway and prevent all clients from connecting to it apart from the repeaters?
Sure, use SSID availability to remove all SSIDs from the gateway. Mesh uses a dedicated, non user facing SSID for the AP to AP connection.
Are there other gateway APs in usable RF range to the repeaters? Repeaters will form mesh connections to the gateway with the best link quality as another member mentioned above. And, repeaters should be able to automatically form a mesh connection to alternate gateways should it's previous gateway become unavailable.
Meraki mesh is auto forming and self healing. I would recommend a Support case so an Engineer can take a closer look at the issue with you.