Gladness, n.
Etymology: [AS. glædnes.]
State or quality of being glad; pleasure; joyful satisfaction; cheerfulness. They . . . did eat their meat with gladness and singleness of heart. Acts ii. 46.
Note: Gladness is rarely or never equivalent to mirth, merriment, gayety, and triumph, and it usually expresses less than delight. It sometimes expresses great joy. The Jews had joy and gladness, a feast and a good day. Esther viii. 17.
Shades of meaning
Fernald has no entry of its own for gladness; he lists it under happiness.
Where he writes about it:
Gladness is happiness that overflows, expressing itself in countenance, voice, manner, and action. — from happiness
Joy is more intense than happiness, deeper than gladness, to which it is akin, nobler and more enduring than pleasure. — from happiness
Gaiety is more superficial than joy, more demonstrative than gladness. — from happiness