It's incredibly impossible to find things for FreePascal because all searches end up at some sort of Delphi related site.
Is there a built-in Dictionary object?
This page references "TDictionary" under the Generic Classes section, but I have no idea what unit it might be in, or if it even exists.
The unit fgl contains the basic generic classes for freepascal.
Among those classes, the closest to a TDictionary is TFPGMap.
An example how to use this class can be found here: Class Map or Dictionary, anyone?.