If you are "capturing" JMS messages via GG, and creating a GG trail -- then, will this trail be applied to a target database, using GG replicat? If so, then run "defgen" against that target database schema, then you have your source-definitions.
The second part of this requires that you have your JMS receiver (aka JMS "capture", aka JMS "VAM") properties file configured so that it parses the incoming XML so that it maps to the target database schema.
The "gendef" (not to be confused by "defgen") would be useful when you have input (JMS messages) generated via (for example) a mainframe application, and already had a cobol copybook formatted definition file, and wanted to convert this "definition" into a GoldenGate source-"definition" file.
(Note: if using GG v12 defgen, be sure to generate the source-definitions file (via defgen) using "format release 11.2" source-definitions output. Because presently the only GG JMS capture version available is as of yet v11.2. But these trails can be sent to a newer, v12 replicat without a problem.)