You will get the symbols on a piece of paper from the gardener when you have done all that he asks you to do. Go to the door with the coded lock and find the symbols that are on the paper. You have to find the symbols in the same order on the door that they are on the paper. It's like a "Find a Word" game but with symbols.
You can see the solution in the video on youtube. Search for Ashley Clark Secret of the Ruby walkthrough part 5 and the solution is near the end of the video at about 6:10.