Although 刚(gāng)才(cái), 刚(gāng) and 刚(gāng)刚(gāng) all means ‘just now’, what they emphasize are different.
刚(gāng)才(cái) is a time noun, means ‘the time shortly after the speech time’, with the opposite meaning ‘now’. While 刚(gāng) is a time adverb, means ‘no long after the event’, with the opposite meaning ‘a long time ago’. 刚(gāng)刚(gāng) can express the meaning of both 刚(gāng)才(cái) and 刚(gāng). For example,
gāngcái wǒ zài shuìjiào， xiànzài kāishǐ xiěwénzhāng
Just now I be doing sleep now start write essay
‘I was sleeping just now, and start to write essay now.’
Wǒ gang qǐchuáng jiù kāishǐ xiěwénzhāng
I just get up start write essay
‘I start to write essay just after getting up.’