You can start Tomcat server in debug mode by either running the Gradle task

gradlew start

or by running the bin/debug.* command file of the installed Tomcat.

After this, the server will accept debugger connections over port 8787. Port number can be changed in the bin/setenv.* file, in the JPDA_OPTS variable.

For debugging in Intellij IDEA you need to create a new Remote type Run/Debug Configuration element in the application project and set its Port property to 8787.