A regular mortgage has its balance decrease over time (you could say it 'amortizes' over time), because the total monthly payments are higher than the monthly interest charge.
Take a 100k mortgage with 3% interest, with a 30 year term. Monthly interest will be about 100k * .03 / 12 = $250, while the monthly payment will be about $420. Therefore every month, $170 is used to pay down the remaining principal balance. In the second month, that remaining principal balance will accrue a tiiiiny bit less interest, and you will have the same $420 payment, so a liiiiittle bit more will be used to pay down principal.
In a GPM, the payment in your first month might only be $200, even though $250 in interest was charged. So your payment won't even cover the interest for that month, and next month you will have a larger principal balance remaining. You could call this negative amortization, for the period until the monthly loan payment is higher than the monthly interest charge.