The Trinidad and Tobago international, who made the move from St Andrews to Highfield Road on Tuesday, gave his new side a 2-0 lead against The Gills with a 49th minute penalty after former Villa defender Steve Staunton had been upended in the box.
Gillingham fought back however - a Patrick Agyemang double earning the Kent side a 2-2 draw.
John has now scored on his debut for all three of the English sides he has played for, Nottingham Forest, Blues and now Coventry.