I've built at least MB's with the GT5, the only thing that makes any sense is the plug worked it's way loose and it just popped that last thread or two.
It is no doubt a slant head and it's not like it's hard to tell, either the spark plug sticks straight up from the head or it slants.
Not you can reverse a slat head so the plug slants forward, I do this on all mine as I like the CDI above the exhaust on the front downbar.
If you want a touch more power this is the best $6.50 you'll ever spend.
An Iridium spark plug, beats the pants off those tri-fire things.
http://www.sickbikeparts.com/catalog/product_info.php?products_id=111
Glad to hear your running.