(gdb) watch function::variable if function::variable == 0xff
(gdb) run
This set of commands, again in a fresh gdb instance, works:
(gdb) info address variable
(gdb) info address function::variable
(gdb) whatis variable
(gdb) whatis function::variable
(gdb) watch function::variable if function::variable == 0xff
(gdb) run
Discuss.