Hi,
I’m a beginner on Graylog.
Since version 2.2.2, Graylog takes native Fortigate logs.
But with versions 2.2.3 and 2.3, I have a problem of date or rather schedule.
My FortiGate is in GMT + 1 but Graylog must work in UTC format.
A solution ? 
Try using a Raw/Plaintext input and extract the relevant parts of the messages using extractors (Grok extractor) or processing pipeline rules.