The “G” is pronounced as in “guitar”. “Goi” is a diphthong in Spanish and it’s pronounced as “boy”. “coe” is not a diphthong (the vowels can be separated) and it’s pronounced similar to “Coen” but without the in-built /ou/ diphthong. “chea” is pronounced with a hard “ch” as in “chair” and it is where the stress falls (on the “e”). It’s a Basque surname but it had already changed by the time my great-grandfather, or great-great grandfather had arrived in Cuba from the Canary Islands. I hope this helps. :-)
Where does your Gigi come from?