See below explanation
Step 1:
Click New found on the upper left-hand corner of the Azure portal.
Step 2:
Select Networking and then select Application Gateway in the Featured list.
Step 3:
Enter these values for the application gateway:
appgw1015 - for the name of the application gateway.
SKU Size: Standard_V2
The new SKU [Standard_V2] offers autoscaling and other critical performance enhancements.

Step 4:
Accept the default values for the other settings and then click OK.
Step 5:
Click Choose a virtual network, and select subnet0.
References:
https://docs.microsoft.com/en-us/azure/application-gateway/application-gateway-create-gateway-portal