According to the Longman Dictionary of Contemporary English, "glad" does not have a comparative form, see glad - Definition from Longman English Dictionary Online. However, other sources indicate that there is one, see Gladder - definition of Gladder by the Free Online Dictionary, Thesaurus and Encyclopedia.
Has Longman made a mistake saying that there is no comparative form?
"Gladder" is fine with me and with several online dictionaries. Having said that, I'm not sure I've ever said it. I just tried to put it in a sentence in my head and it sounded rather odd. I would probably use:
I am more glad than you could possibly imagine.
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