"It is not really cloudy."

Translation: Chan eil i gu math sgòthach.

[JaronOfWaterdeep]

'Gu math' it seems is an intensifier used with adjectives, meaning 'very.'

'Gu mòr' is also an intensifier, but so far I only remember seeing it with verbs. Specifically in expressions of taste/opinion. 'Is toil leam càise gu mòr.'

Could one please summarize the boundaries between each one's use?

[artemis084]

'Gu mòr' means 'a lot', I think. The Gaelic use of 'gu mòr' and 'gu math' seems to equal the English use of 'very' and 'a lot'. As you said, 'gu math'/'very' is used with adjectives and 'gu mor'/'a lot' is used with verbs.

I don't know this too well either, but maybe it helps.

[SleepyLass]

Why is glè not allowed here?

[Medusa_--]

Same question, why was glè wrong?