A kusto query which can be run under the Azure Resource Graph Explorer (https://portal.azure.com/#view/HubsExtension/ArgQueryBlade) to get a list of hostnames/listeners configured on all app gateways under your subscriptions.

Kusto: Find all Azure App Gateway Listeners & HostNames

(
    resourcecontainers
    | where type =~ 'microsoft.resources/subscriptions'
    | project SubscriptionName=name, subscriptionId
) | join kind = inner
(
    resources
    | where type =~ 'microsoft.network/applicationgateways'
    | project subscriptionId, AppGateway=name, httpListeners = properties['httpListeners']
) on subscriptionId
| mv-expand httpListeners
| project SubscriptionName
, AppGateway
, Listener = httpListeners.name
, Protocol = httpListeners.properties.protocol
, SingleHostName = httpListeners.properties.hostName
, HostNames = httpListeners.properties.hostNames
| mv-expand HostName = coalesce(SingleHostName, HostNames)
| project-away SingleHostName, HostNames