To check if a folder exists or not, you can simply use the exists()
method:
// Create a File object representing the folder 'A/B'
def folder = new File( 'A/B' )
// If it doesn't exist
if( !folder.exists() ) {
// Create all folders up-to and including B
folder.mkdirs()
}
// Then, write to file.txt inside B
new File( folder, 'file.txt' ).withWriterAppend { w ->
w << "Some text\n"
}