Here is how to enabled GDB debugging with VMWare for Cosmos.
Normally we do low level GDB debugging with QEMU, but under Vista and Windows 7, GDB with QEMU is basically broken (easy fix though if you don't mind building it). So here is how to enable GDB for VMWare Workstation. Create a new VMWare vmx file and set it to boot your Cosmos .iso image. While the VM is powered off, edit the .vmx file in notepad and add these lines:
debugStub.listen.guest32 = "TRUE"
debugStub.hideBreakpoints = "TRUE"
monitor.debugOnStartGuest32 = "TRUE"
Now when you start Cosmos, VMWare will break on the first instruction (BIOS, not Cosmos yet) and wait. It will listed on port 8832 for a GDB client.
More information on the debugstub setting. There is also a debugStub.hideBreakpoints option, although information about it is scarce. I've set it to true, because several sources say to set it to true for various GDB clients.