Greetings, hello, Can somebody help me with the true etymology (meaning) of the Gladys name?

Best regards British friends. I wish to ask to somebody who knows it, the true etymology (meaning) of the name Gladys. That your sources be as trustworthy as possible, please, because (in Internet) there is a chaos of several etymologies for the name Gladys. I have a relative of nearly 63 years old with this name. Thank you very much.