Welcome to Flyertalk!
Gift cards and cash equivalents are not supposed to count towards spending for your welcome bonus, so it's not worth risking. Amex has a whole team called the "RAT" that watched for what they consider ineligible charges. If you read on flyertalk there is a whole forum for "MS" or "Manufactured Spending" with tips like prepaying your federal taxes (there is a fee for doing this).
Google tells me your Amex welcome bonus should be 5,000 MQM if you spend $1000 in the first three months. Do you have that bonus, and if so is that accounted for in your 6k shortage, or would you actually be only 1k shy of silver medallion after the sign up bonus? Do you have any existing flights booked you can upgrade to first class (for the 50% MQM bonus)?