刚 vs 刚才
gāng vs gāng cái
Comparing "gang" and "gangcai"
刚 is an adverb glued to the verb; 刚才 is a time word that can move around.
刚 = adverb, before the verb · 刚才 = time word, movable
How it works
你刚才去哪儿了？我刚回来 — where did you just go? I've just got back.
Because 刚才 is a time word, it can open a sentence: 刚才有人找你. 刚 cannot.
刚 also means the action was recent relative to something else, so it can appear in sentences about the future: 我刚到就下雨了.
Examples
nǐ gāng cái qù nǎr le?
你刚才去哪儿了？
Where did you go just now?
wǒ gāng huí lái
我刚回来。
I've just got back.
gāng cái yǒu rén zhǎo nǐ
刚才有人找你。
Someone was looking for you a moment ago.