As like army and navy,candidates for garud commando are not recruit  from volunteer or other branches.selection is done through airmen selection centers by exams ,physical and medical.
After selection,candidate are put for rigorous training.candidate have only one chance to become a garud trainee.once candidate complete basic training and meets to certain standard,they sends to commando force and retained in this stream throughout the career.
Thanks,i hope it is helpful for you.