ppm means 1 part out of a million. It could be on a weight basis or on a volume or molar basis. In this case I presume it is on a weight basis.

1 gram per metric tonne equals 1 ppm (assuming weight basis)

1 tonne has 1000 kg each has 1000 grams, so 1 tonne has 1 million grams, therefore 1 g/t = 1ppm