If the datatype of EVENT_GMT_TIMESTAMP is a TIMESTAMP, it's simple Standard SQL:
CAST(EVENT_GMT_TIMESTAMP AS DATE)
CAST(EVENT_GMT_TIMESTAMP AS TIME)
If it's a CHAR you need to apply a FORMAT, too:
CAST(CAST(EVENT_GMT_TIMESTAMP AS TIMESTAMP FORMAT 'dd/mm/yyyyBhh:mi:SS.s(6)') AS DATE)
CAST(CAST(EVENT_GMT_TIMESTAMP AS TIMESTAMP FORMAT 'dd/mm/yyyyBhh:mi:SS.s(6)') AS TIME)
Edit:
For simply changing the display format you need to add a FORMAT and a CAST to a string:
CAST(CAST(EVENT_GMT_TIMESTAMP AS FORMAT 'YYYYMMDDHHMI') AS CHAR(12))
or
CAST(CAST(EVENT_GMT_TIMESTAMP AS FORMAT 'YYYYMMDDHHMISS') AS CHAR(14))
If you don't care about display, just want to truncate the seconds:
EVENT_GMT_TIMESTAMP - (EXTRACT(SECOND FROM EVENT_GMT_TIMESTAMP) * INTERVAL '1.000000' SECOND)
Working with timestamps is a bit tricky :-)