Hi Andrew,
I don't believe there is any function in MYSQL (at the moment) to automatically convert a timestamp to GMT time on insert.
A possible way to tackle this issue, if you can't change the MYSQL config file, is:
INSERT INTO $myTable VALUES( DATE_ADD( NOW(), INTERVAL -3 HOUR));
The problem with the above statement, is you need to know the hour adjustment to GMT from your server's time. If you know this, just replace -3 with the adjustment factor. Additionally, replace the NOW() value (which just inserts the current date) with any date/time value you want to insert.
Cheers,
Ash