The following should suffice:
def array = [0, 0, 0, 0, 0] as byte[]
Have a look here for more details on arrays in groovy.
In addition to rich.okelly's answer,
byte[] array = [0, 0, 0, 0, 0]
works as well
You can't initialize a literal array the same way because Groovy thinks the curly brackets form a closure. What you want is something like
def x = [ 0, 0, 0, 0, 0 ] as byte[]
See more: here