Can anyone please tell me why, when I do a simple system update/upgrade with Gslap, certain kernels and packages show as available but are 'excluded'..?
Because upgrading these packages unwittingly can break a lot of things, including but not limited to booting the system.
You can instead upgrade these packages using slapt-get from the command line.
If you really want to use gslapt to upgrade these packages and are aware of the problems that can arise and how to avoid and/or fix them, feel free to remove the exclude expressions in the gslapt preferences.
If using the lilo bootloader, don't forget to run sudo lilo -v after updating your kernel.