For the 'ibmint generate tests' to generate a test it needs (at least) 2 messages, the input message for the message flow node, and the 'expected' output message from the message flow node. The test which is generated invokes the message flow node using the input message and then compares the actual output message with the expected message as read from the file. (btw. there is a recent report of the error handling not being perfect in the ibmint generate tests command which will be fixed in a forthcoming fix pack so it is likely the exception you encountered will be fixed by this update).
Hopefully this helps, as for the next steps it depends on what you want to do. If you want to write tests in the toolkit and then export the test project for it to be built outside the toolkit then yes you can copy the gradle build files from a test project created by the "ibmint generate tests" command into the exported test project and use them to build it.