The rule I learned for the pronunciation of the digram "gu" before a vowel in Latin was /gw/ after "n", vs. g + vocalic u anywhere else. But I just discovered the exception urgueo /urgweoː/. This is a variant of the verb urgeo. A search of the PHI Latin texts corpus for #urgu shows fairly wide usage, so I think that urgueo was not just a marginal or late variant.* I can't figure out why gu is present here; can anyone explain it?
The form urgueo doesn't seem to be discussed in de Vaan 2008, and the etymology given there for urgeo doesn't provide any explanation for the presence of an alternative form with a labiovelar: de Vaan says urgeo is from a PIE root with ǵʰ.
I am aware of third-conjugation verbs that also show variation between stem-final "g" and "gu", but those make more sense to me because the stem-final consonant is in some inflected forms brought before a rounded vowel, which is a context where sound changes neutralizing the contrast could occur. For example, de Vaan says stinguo comes from a root *stengʷ-, and that alternative forms with plain g like stingo, stingere can be explained as caused by the development of stingunt as a simplified form of stinguunt.
De Vaan says that the forms tinguo (alongside tingo) and coinquo have non-original /w/ by analogy, but I think that this analogy would have been helped by the neutralization discussed in the preceding paragraph. I've seen other sources that say that ungo existed as a (non-etymological) variant of unguo; like the previously discussed examples, this is a third-conjugation verb.
De Vaan indicates that in langueo, the only other second-conjugation verb that I know of ending in -gueo, the gu was derived from PIE *ǵ followed by *u. That doesn't seem to help much in explaining urgueo, unless there is a some way that an optional *u after the g could be included in this word's etymology.
Was urgeo close enough in meaning to stinguo for the latter to have influenced the form of the former, even in the absence of any phonetic factor facilitating the replacement of g with gu between r and e? That seems odd to me, but maybe it's the best explanation. Is anyone familiar with a discussion of how this variant might have arisen?
*If anyone can better inform me about the distribution of urgeo vs. urgueo, I would appreciate it. I know that scribes sometimes altered the spelling of words when copying texts, but based on what I've seen so far, I don't think urgueo can just be attributed to postclassical scribes. It does strike me as a little odd that the works of Cicero and Ovid contain both g and gu forms.