build.gradle

project.ext.files = []

tasks.addRule('Use files as arguments') { taskName ->
    if (!taskName.equals('dosomething') && file(taskName).isFile()) {
        files.add taskName
        task(taskName) // Define task to prevent task-not-found error
    }
}

task dosomething << {
    println 'do something'
    println files
}

result1.log

$ gradle dosomething files/a.txt files/b.txt
:dosomething
do something
[files/a.txt, files/b.txt]
:files/a.txt UP-TO-DATE
:files/b.txt UP-TO-DATE

result2.log

$ gradle dosomething files/a.txt files/b.txt -q
do something
[files/a.txt, files/b.txt]