Gather is about populating the event log variable in the assembly flow, so that you can then customise it in some way.
Send is about transmitting that data to analytics.

The data is automatically sent to analytics at the end of the assembly flow without even having a Log policy in send mode, so chances are you dont want to do Send, since it will lead to double accounting in analytics.

Here's an example of how to use the Log policy to customise the data sent to analytics: https://www.ibm.com/docs/en/api-connect/10.0.8?topic=data-customizing-gateway-script-set-variable-operation

Note: if adding custom data to the event log record send to analytics, please do it within the custom_data object otherwise you won't be able to query for that data via the Analytics UI/API.

okay.. but when I tried with Gather-Only, Gather-and-Send mode I can see the details in analytics log. However, the call is not being logged when Send-Only option is selected

Gather is about populating the event log variable in the assembly flow, so that you can then customise it in some way.
Send is about transmitting that data to analytics.

The data is automatically sent to analytics at the end of the assembly flow without even having a Log policy in send mode, so chances are you dont want to do Send, since it will lead to double accounting in analytics.

Here's an example of how to use the Log policy to customise the data sent to analytics: https://www.ibm.com/docs/en/api-connect/10.0.8?topic=data-customizing-gateway-script-set-variable-operation

Note: if adding custom data to the event log record send to analytics, please do it within the custom_data object otherwise you won't be able to query for that data via the Analytics UI/API.