In a medium pot, heat mushroom soup. When soup is hot, stir in the cream cheese; continue stirring until all of the cheese has melted.
Stir gelatin into the cheese and soup mixture. Stir constantly, being careful not to let the mixture come to a boil. When the gelatin has dissolved, remove the pot from the heat and let cool.
Mix celery, onion, and mayonnaise into the cooled soup mixture. Add crabmeat to the mixture. Pour the dip into a serving bowl, cover the bowl, and refrigerate it for 2 hours before serving.
