Hi Rohit,
You need to create an A record for every subdomain in your DNS. This article has similar example but the A record is created using the root domain.
In your case it would look like this
mail.mydomain.com 13999 IN A a.b.c.d
git.mydomain.com 13999 IN A w.x.y.z
etc.
If you are using Cloud DNS the following discussion will interest you.