It says on google you can use okra as a substitute - plenty of that around lol.
Most gumbo recipes use two types of thickeners: a roux and either file powder or okra. File powder is ground sassafras leaves and adds a distinctive root beer flavor to gumbo. If okra is used, it should be cooked for approximately 45 minutes to lose its slimy texture, while file powder should be added at the end of cooking. It is recommended that okra and file should not be used in the same batch of gumbo, as the gumbo may become too thick.
Matilda