There are no official APIs for users.
Which means we're stuck using the generic-object API.
The following one-liner should get you what you need in CSV format:
SID=`mgmt_cli -r true login --format json | jq '.sid' -r`; mgmt_cli --session-id $SID show generic-objects class-name "com.checkpoint.objects.classes.dummy.CpmiUser" --format json | jq '.objects[].uid' | while read X; do mgmt_cli --session-id $SID show generic-object uid $X --format json | jq '[.cpmiName, .email] | @csv' -r; done; mgmt_cli --session-id $SID logout > /dev/null
This creates a session (using -r true), gets all the users (up to 500 I think) and queries each use record (by uid) to get the name and email address, then logs the session out.
Output is in CSV format.