The spelling of the word "Gaddingly" may seem confusing at first glance, but it can be easily understood with the help of IPA phonetic transcription. The correct pronunciation of this word is /ˈɡædɪŋli/, with stress on the second syllable. The "dd" in the first syllable is pronounced as a voiced alveolar plosive, similar to the "d" in "day". The second syllable contains the sound "ɪŋ" which is pronounced as a nasalized "ih" sound followed by a voiced velar nasal. With this knowledge, the spelling of "Gaddingly" becomes clearer and easier to remember.
In a gadding manner.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.