Hi Manoj
I understand that this is confusing.
Yes, the SQL Server is a standard, free connector. However, the use of the on-premises gateway requires a P1 or P2 license. Therefore by extension of this rule, if you want to connect to an on-premise SQL Server through a gateway, you'll need a P1 or P2 license.
If for example you were to connect to a SQL Server Azure database instead, this would not require a P1 or P2 license because this would not require the on-premises gateway.
Hope that makes a bit more sense for you!