Learn English – reason why “gn” in “reigning” is pronounced /n/ while in “regnant” it is pronounced [gn]

There are lots of similar examples with a different consonant: "thistle", "bustle", "whistle", "castle", and so on. I suspect that at some point during the history of English, there was a general sound change: -stle → -sle, and -scle → -sle. It just happened that the only moderately common words that ended with -scle were muscle and corpuscle. In neither of them is the "c" currently pronounced.

The reason that the "c" is still pronounced in "muscular" is that this sound change only happened at the end of a word; it's the same reason the "t" is still pronounced in "castellan".

There are also the rare words "crepuscle" (akin to "crepuscular"), which means twilight, and "arbuscle" (found in Johnson's 1795 dictionary), which meant a small tree or shrub. These words are quite rare nowadays, and I can't find a definitive pronunciation for them on the web.

Per the Naval Historical Center:

The English borrowed the word "sergeant" from the French in about the Thirteenth Century. They spelled it several different ways and pronounced it both as SARgent and SERgeant. The latter was closer to the French pronunciation. The SARgeant pronunciation became the most popular, however, so that when the Nineteenth Century dictionary writers agreed that the word should be spelled "sergeant" they could not change the popular pronunciation. Thus, we say SARgeant while the French and others say SERgeant.