c. 1300, "to be glad;" 1550s, "to make glad;" see glad (adj.) + -en (1). Earlier in both senses was simply glad (v.), from Old English gladian, Mercian gleadian "be glad; make glad." Related: Gladdened; gladdening.
Old English glæd "bright, shining, gleaming; joyous; pleasant, gracious" (also as a noun, "joy, gladness"), from Proto-Germanic *gladaz (source also of Old Norse glaðr "smooth, bright, glad," Danish glad "glad, joyful," Old Saxon gladmod, in which the element means "glad," Old Frisian gled "smooth," Dutch glad "slippery," German glatt "smooth"), from PIE root *ghel- (2) "to shine." Apparently the notion is of being radiant with joy; the modern sense "feeling pleasure or satisfaction" is much weakened. Slang glad rags "one's best clothes" first recorded 1902.
word-forming element making verbs (such as darken, weaken) from adjectives or nouns, from Old English -nian, from Proto-Germanic *-inojan (also source of Old Norse -na), from PIE adjectival suffix *-no-. Most active in Middle English and early modern English, hence most verbs in -en are comparatively recent.
<a href="https://www.etymonline.com/word/gladden">Etymology of gladden by etymonline</a>
Harper, D. (n.d.). Etymology of gladden. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/gladden