"A mouse is very small."
Translation:Tha luch glè bheag.
What I'm getting from here is that gu in front of various words change their meaning slightly, so that's when gu is used. Glè intensifies the attached word without otherwise altering the meaning
What they're saying here makes sense, that gu snog is different than glè snog in that saying something nicely is different than saying it very nice
Someone chime in if I'm glè wrong about everything
glè = very, gu math on its own = well, but also = quite when it is used to modify an adjective - tha luch gu math = a mouse is well, but tha luch gu math beag = a mouse is quite small vs. tha luch glè bheag = a mouse is very small.
In general, gu + adjective (used to modify a noun) = adverb (used to modify a verb). tha Calum luath = Calum is quick (describing Calum), tha Calum a' coiseachd gu luath = Calum is walking quickly (describing walking).
When an intensifier is added to an adverb, the gu usually drops out - so glè mhath could mean "very good" or "very well" depending on context. Tha thu a' bruidhinn gu luath - you are speaking quickly, but Tha thu a' bruidhinn ro luath - you are speaking too quickly, rather than tha thu a' bruidhinn gu ro luath, which would be very rare.