IPA: /ˈɡlædər/
KK: /ˈɡlædər/
Feeling or showing joy or pleasure; happy.
She was gladder than ever when she heard the good news.
Gladder is formed from "glad" (from Old English "glæd", meaning joyful or happy) and the suffix "-er" (meaning one who). The word "gladder" means one who is more joyful or happy than another.
Think of someone who is 'more joyful' — 'glad' means happy, and '-er' indicates a person or thing that has more of that quality.