The registry lets you set custom flags and keywords per
message. You can use the Gnus->Registry Marks menu or the
M M x keyboard shortcuts, where x is the
first letter of the mark's name.
The custom marks that the registry can use. You can modify the default list, if you like. If you do, you'll have to exit Emacs before they take effect (you can also unload the registry and reload it or evaluate the specific macros you'll need, but you probably don't want to bother). Use the Customize interface to modify the list.
By default this list has the
Important,Work,Personal,To-Do, andLatermarks. They all have keyboard shortcuts like M M i for Important, using the first letter.