You can run SmartEvent on a standalone gateway if you have enough memory (at least 16 GB).
There are a few limitations documented in the various release notes around standalone mode.
The main issue with standalone is the resources needed to run both management and gateway functions on the same system.
Management is very disk I/O intensive and memory intensive, gateway is network I/O intensive.
You can expect, at a minimum, that a gateway configured as standalone will have a 50% penalty compared to the same gateway deployed with separate management.