I'm looking to convert the results for these fields in PST time zone, so that I can fetch the events based on these fields but not based on the event timestamp. Below two fields are in GMT format. Please help
FIRST_FOUND_DATETIME="2021-12-12T08:30:29Z",
LAST_FOUND_DATETIME="2021-12-20T16:12:28Z
Don't worry about it. Once you convert the fields into epoch form so you can compare them they will be in epoch form. When you then format them for display they'll be in your selected time zone.
For example:
index=foo
| eval First=strptime(FIRST_FOUND_DATETIME, "%Y-%m-%dT%H:%M:%S%Z"), Last=strptime(LAST_FOUND_DATETIME, "%Y-%m-%dT%H:%M:%S%Z")
| where First <= relative_time(now(), "-7d") AND Last <= relaive_time(now(), "-6d")
...