Keep-Alive timeout governs how long the Application Gateway waits for a client to send another HTTP request on a persistent connection before reusing it or closing it. TCP idle timeout governs how long a TCP connection is kept open if there's no activity.
The Keep-Alive timeout in the Application Gateway v1 SKU is 120 seconds and in the v2 SKU it's 75 seconds. For private IP addresses, the value is nonconfigurable with a TCP idle timeout of 5 minutes. The TCP idle timeout is a 4-minute default on the frontend virtual IP (VIP) of both v1 and v2 SKU of Application Gateway. You can configure the TCP idle timeout value on v1 and v2 Application Gateways to be anywhere between 4 minutes and 30 minutes. For both v1 and v2 Application Gateways, you need to navigate to the public IP of the Application Gateway and change the TCP idle timeout under the "Configuration" blade of the public IP on Portal. You can set the TCP idle timeout value of the public IP through PowerShell by running the following commands:
$publicIP = Get-AzPublicIpAddress -Name MyPublicIP -ResourceGroupName MyResourceGroup
$publicIP.IdleTimeoutInMinutes = "15"
Set-AzPublicIpAddress -PublicIpAddress $publicIP
For HTTP/2 connections to the frontend IP address on Application Gateway v2 SKU, the idle timeout is set to 180 seconds and is nonconfigurable.