When an UE measures serving cell is better than the threshold configured, it reports A1.
Event A1 has following entering and leaving conditions:
Entering condition: MEASserv - Hyst > threshold
Leaving condition: MEASserv + Hyst < threshold
If you go through the measConfig which is sent as part of RRC Connection Reconfiguration message.
Measurement gap is used when eNodeB asks UE to measure frequencies other serving. As per my understadning, there is no relation of measurement gap and event A1. Due to load balancing, serving eNodeB can ask to measure inter frequency when it decides to move few connected users to another frequency. Another scenario, when UE is moving away from serving cell and serving cell doesn't have another eNodeB running on same frequency as a neighbor. In this case also, eNodeB ask for inter-frequency measurement.