You could do it using the java syntax, e.g.:
Properties props = new Properties()
InputStream ins = new FileInputStream("/path/file.properties")
props.load(ins)
ins.close()
This should work in any groovy script. There might be a more “groovy” way of doing it though, using closures or some other fancy shortcut.
EDIT: “in” is a reserved word in groovy. A variable can’t be named that way, renaming it to “ins”