What is the name of Proto-Germanic e/ē lengthening?

Question

What is the name of Proto-Germanic e/ē lengthening before ō?

*kwelaną *kwēlō
*wrekaną *wrēkō
*sprekaną *sprēkō
*frehnaną *frēgō

I know that e/ē lengthening before "i" is Vṛddhi gerundive?
*nemaną *nēmiz
*kweþaną *kwēdiz
*wrekaną *wrēkiz

Answer 1

There are hundreds of sound laws associated with any language, they won't all get their own pithy name. This is just "the lengthening that happens when an ō-stem noun is derived from a strong verb". You might call this a vṛddhi-derivation (vṛddhi being a Sanskrit word used for the strongest grade of a vowel in a given system of vowel gradation), but that's a broader category of things than just lengthened ō-stems derived from strong verbs.

"Vṛddhi gerundive" isn't the name for lengthening of *e before *i; *nēmiz, *kwēdiz, &c. are gerundives (a type of verbal adjective), and they're called vṛddhi because they have a lengthened grade in the root. This lengthening is part of the process of derivation, not phonologically conditioned by the following *i: cf. the vṛddhi-gerundives *lēgaz from * ligjaną ~ legjaną, *frōdaz from *fraþjaną, *gōdaz from *gadaną, &c., all without the *i.