As the others said my love the random blood test is an indicator. So, follow the dietary instructions prior to the GTT for the most accurate result.
Also, many women develop GD and they have great diets and are not overweight (ME!!!). Other women can be very overweight and eat abominably and sail through. As Falguni said its a bit hit and miss there.

It is only a small proportion of women with GD that require insulin so don't put the cart before the horse. Just one step at a time. Book in for that test and I hope very much it's all good for you...