"It was really bright yesterday."
Translation:Bha i gu math soilleir an-dè.
Gu math 'quite, really' is an adverb, and, just as in English, adverbs go before the adjective, although it can go after other parts of speech.
Mark gives about 50 examples with an adjective, and in all cases the gu math goes in front. He also gives about 30 non-adjectival examples where it can go after the word it qualifies. Note in particular that he gives three examples after a verbal noun, where it would go in front in English because the English translation uses a participle (which is an adjective)
bha e air a phiobrachadh gu math – it was well peppered
I can't find this in any dictionary, but I would actually accept your sentence but with a different meaning: 'It was bright indeed yesterday', thus emphasising the fact of it being bright, rather than how bright it was. I wonder if anyone else agrees with me?