The syntax of the directive is
!GCC$ unroll N
You can use this directive to control how many times a loop should be unrolled.
It must be placed immediately before a DO loop and applies only to the
loop that follows. N is an integer constant specifying the unrolling factor.
The values of 0 and 1 block any unrolling of the loop.
For DO CONCURRENT constructs the unrolling specification applies
only to the first loop control variable.