What I have found in this document mentions VPC gateway are free of charge interface type charges by hour of usage.
Also I know gateway only supports S3 and DynamoDB and interface supports many services. My question is if gateway is free of charge for S3/DynamoDB, why anyone wants to use interface type ? Is there any other reason ?
There are a number of specific use cases and functionalities that a VPC interface endpoint offers over a free gateway endpoint. One example is that Gateway does not allow access from on prem or from other AWS regions.