Miter gauges are usually made to fit a 3/4" slot in the table saw. This is the industry standard. Unfortunately, some Ryobi saws have an undersized T slot. This means that all the accessories and miter gauges on the market cannot be used on these table saws. When I bought mine, I never thought to think it wouldn't be the standard slot. Why Ryobi did this? Who can say.
Replacement miter gauges can be found on Amazon.