To the [i] sound, add ει and οι.
As for the sound of γ: the comment by affitro here is very helpful. The audio is correct. The sound in this case is similar to the French r.
I had found some time ago a very clear guide in Duolingo which explained exactly how to pronounce each sound in Greek, but unfortunately I have lost it...
Γλυκός is an adjective that means sweet.
The neuter form of the adjective, γλυκό (pl. γλυκά) is used as a noun, meaning any sweet food. So γλυκά is not sweets, as in candy.
The google image results of κέικ is really what is called a κέικ in Greece. The word for cake, as in birthday cake, is τούρτα (try googling that too).
Γλυκά encompass everything: cake, κέικ, chocolate, candy, preserves, delights, and many more traditional γλυκά, some of Turkish origin, that aren't known in the West.
The word for "dessert" is " επιδόριο". Just "γλυκά" would be more descriptive of what is being served.
Just to whet your appetite