Adding priority to the task permanently on Linux machine

Adding priority to the task permanently on Linux machine

How do I increase the priority for a task in Debian Linux system? After a
little research I found that I can increase the priority to a task using
nice command and if I want to increase the priority for an existing task
then I can use the renice command.
In some tutorials its mentioned that, nice values vary from 0 to 99, But
in some tutorials states that nice values vary from -20 to 19. Which one
is correct?
How do I increase the nice value for an GUI task? If I am doing a copy
using a terminal then I can increase its priority using nice command, but
how do I increase the priority of a task done using GUI (for example:
copying files from USB to System through GUI)
Is it possible to set the priority of a task every time it is run? I want
to set highest priority for copying files via the GUI permanently.