Eh, sorry it's not your fault at all, but I simply just can't wrap my head around the idea that ppm = (gs/gs) * 10^6 is significantly different from 1 ppm = (gs/gs) * 10^6.
It seems like wikipedia defines ppm as a dimensionless quantity (like mass ratio) times 10^-6. It doesn't even seem that popular of a notation in chemistry anyways. Either way I think ppm = 1 mg/kg would be a less confusing formula for the purposes of the MCAT, if it is even tested.
Think about percents.
To determine mass percent we divide the mass of the solute by the mass of the solvent and then multiply by 100.
This gives us the percent which is the same as saying parts per hundred. If the mass of the solute is 1% of the solution mass, then for every 100 grams of solution, we have 1 gram of solvent.
Apply this to parts per million.
Another way to think about it is to think of these 3 percents. 0%, 50%, 100%
Obviously if solute is 0% of the solution then it is 0ppm
Obviously if solute is 50% of the solution then it is 500,000ppm
Obviously if solute is 100% of the solution then it is 1,000,000ppm
Does grams solute/grams solution*10^-6 get these obviously true results? No. In the case of the solution being 100% solute, the formula you have offered will tell you that there is 1 part solvent per million parts solution.
Does grams solute/grams solution*10^6 get these obviously true results? Yes. No. In the case of the solution being 100% solute, the formula EK has offered will tell you that there are 1 million parts solvent per million parts solution.
EK's formula is correct.