Gladys was born Gladys Christina Coulton, mother’s maiden name Coulton in St Olave (Bermondsey) in 1914. The birth was registered in that district in the third quarter of 1914 (July to September), volume 1d page 289.
Because there is no separate mother’s maiden name it is likely she was born out of wedlock.
You can use the reference details we have given you to order her birth and marriage certificates. Between them those two certificates should tell you who she was born to and what her circumstances were at the time of her marriage. Order from the GRO online, for which the link has already been given at reply #12.
Legal (formal) adoption was not introduced in England until 1927, so she would not have been adopted in the modern sense as an infant, and it is likely any arrangements made were entirely informal.