"The spider and the cat are gray."
Translation:Η αράχνη και η γάτα είναι γκρι.
Yes, but because djectives are also conjugated, you should use γκρίζες (feminine nominative plural) because both αράχνη & γάτα are feminine nouns.
As correct answer 'γκρίζα' is shown. Are both forms (singular and plural ending) possible?
Shouldn't we say "κι η γάτα" or it's same, because normally we say "κι" before a vowel.
Και is also correct. It's not necessary to change it to κι before a vowel, but of course in spoken Greek it's usually 'κι' in those cases, as you say. I don't know if κι has been added as an answer in all (or any?) of the sentences where it may appear, but I would think not because it is too much to expect all spoken variations to be included in Duo's system (same with all the vowel omissions and apostrophe phenomena). Sticking with the formal language pays off here! :D