If you want to use gnumach's built-in debugger (aka ddb and sometimes called kdb), then the thing to do is start the kernel with the -d switch (in a kernel compiled with --enable-kdb). Then you should get the debugger prompt immediately on boot. You can then set breakpoints whereever you like to stop and examine the kernel. Note that this is a primitive debugger, not a source-level debugger, and in fact we don't even support getting an ELF symbol table into the kernel debugger so you don't get any symbols at all.
If you use oskit-mach, it supports (that is, the oskit supports) using gdb over a serial line from another machine. That is by far the best way to debug the kernel. You use gdb and have nice source-level debugging and most everything you are used to from using gdb on user programs. If you can do this (i.e. have two machines and a serial cable) and want to, then you should get started first by debugging some oskit example kernels using gdb over your serial line. There is a debian package of oskit that you can easily install to build your kernels, though I would recommend compiling it yourself from source with -g. You should refer to http://www.cs.utah.edu/flux/oskit/ for information about the oskit and how to set things up to use gdb over a serial line (or at least where to ask for advice, which is not here). Once that is all working for you and you have gotten a feel for using gdb with an oskit kernel, just build oskit-mach with -g and you can debug it like other oskit kernels.
