Gather ExpressRoute Circuits with Global Reach enabled. This does account for multiple subscriptions as well.

Get-AzureGlobalReachEnabledERCircuits.ps1

#Requires -Modules az
Connect-AzAccount
$ERGREnabled = @()
$GRDataFull = @()
$ERCircuits = @()
$Subs = Get-AzSubscription
Foreach ($Sub in $Subs){
    Select-AzSubscription $Sub
    $ERCircuits += Get-AzExpressRouteCircuit
}
Foreach ($ERCircuit in $ERCircuits){
    If ($ERCircuit.GlobalReachEnabled -eq $true -or ($ERCircuit | Get-AzExpressRouteCircuitConnectionConfig))
    {
    $Data = "" | Select-Object SubId, ERResourceGroup, ERCircuitName, GRName, GRAddressPrefix, PeerERResourceGroup, PeerExpressRoute, PeerSubId, SPName, PeeringLocation, Bandwidth, GlobalReachEnabled, ServiceKey
    $Data.SubId = $ERCircuit.Id.split("/")[2]
    $Data.ERResourceGroup = $ERCircuit.Id.split("/")[4]
    $Data.ERCircuitName = $ERCircuit.Name
    If ($ERCircuit | Get-AzExpressRouteCircuitConnectionConfig -ErrorAction:SilentlyContinue)
    {
    $GRData = ($ERCircuit | Get-AzExpressRouteCircuitConnectionConfig -ErrorAction:SilentlyContinue)
    $Data.GRName = $GRData.Name
    $Data.GRAddressPrefix = $GRData.AddressPrefix
        If ($GRData.PeerExpressRouteCircuitPeering.count -gt 1)
        {   $Data.PeerERResourceGroup = "Multiple Objects"
            $Data.PeerExpressRoute = $GRData.PeerExpressRouteCircuitPeering
        }
        Else {
            $Data.PeerERResourceGroup = $GRData.PeerExpressRouteCircuitPeering.Id.split("/")[4]
            $Data.PeerExpressRoute = $GRData.PeerExpressRouteCircuitPeering.Id.split("/")[8]
        }
        $Data.PeerSubId = $GRData.PeerExpressRouteCircuitPeering.Id.split("/")[2]
    }
    Else 
    {
        $Data.GRName = "No Data Found - Possibly Customer Initiated"
        $Data.GRAddressPrefix = "No Data Found - Possibly Customer Initiated"
        $Data.PeerERResourceGroup = "No Data Found - Possibly Customer Initiated"
        $Data.PeerExpressRoute = "No Data Found - Possibly Customer Initiated"
        $Data.PeerSubId = "No Data Found - Possibly Customer Initiated"
    }
    $Data.SPName = $ERCircuit.ServiceProviderProperties.ServiceProviderName
    $Data.PeeringLocation = $ERCircuit.ServiceProviderProperties.PeeringLocation
    $Data.Bandwidth = $ERCircuit.ServiceProviderProperties.BandwidthInMbps
    $Data.GlobalReachEnabled = $ERCircuit.GlobalReachEnabled
    $Data.ServiceKey = $ERCircuit.ServiceKey
    $ERGREnabled += $Data
    $GRDataFull += $GRData
    }
}
$ERGREnabled #For summarized human readable data
#$GRDatafull #For the raw data if you want to hurt yourself.
<# Example Summarized Return
SubId               : xxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
ERResourceGroup     : naka
ERCircuitName       : naka-er
GRName              : naka-gr-to-zsoldier
GRAddressPrefix     : ###.###.###.##/##
PeerERResourceGroup : zsoldier
PeerExpressRoute    : zsoldier-er
PeerSubId           : xxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
SPName              : Megaport
PeeringLocation     : Las Vegas
Bandwidth           : 1000
#>