As long as the following conditions are met, the GS champion rule can be applied immediately:
1. All other GS champions are confirmed in the top 7 and cannot possibly drop lower than 7th place.
2. This particular GS champion is in 9th to 20th position and cannot possibly drop below 20th position.
If I remember correctly, in 2001, Goran confirmed his place in the TMC quite early, before the Paris tournament, as he had won Wimbledon and was in 12th or 13th place and could not fall out of the top 20. In addition, the year's other GS winners (Agassi, Hewitt and Kuerten) had already confirmed their places.