Using the amazing GDBStub Library for Teensy 3/4 by Fernando Trias, you can now have the GDB experience on Teensy 3/4 without any additional hardware!

With the Teensy servicing multiple Serial Ports you don't lose your Serial output or need extra hardware.

This has been integrated into Visual Micro with the familiar menu options used for Serial and other Hardware Debugging in our latest release 20.06.18.2.
Click image for larger version. 

Name:	TB_Teensy4x-GDBStub.png 
Views:	10 
Size:	2.5 KB 
ID:	20688

All Standard GDB Features are readily available:
Step by Step execution using breakpoints, with navigational Into / Over / Out functions as well
Add breakpoints dynamically at runtime without rebuild / upload
Conditional breakpoints
Editable Watch / Local / Autos Windows
Call Stack Tracing
And More...

Further Information and a Video Example of this GDB Stub in Action in Visual Micro are below:
GDBStub Arduino Library for Teensy 3/4 by Fernando Trias

Visual Micro VS Extension Website

Visual Micro Documentation

You Tube Video of Teensy GDB Stub Setup + Debugging in Action in Visual Micro

Click image for larger version. 

Name:	Debug-Break-TEENSY4-GDBSTUB-VSOnly.jpg 
Views:	19 
Size:	119.0 KB 
ID:	20689