You can either say "Η αρσενική αρκούδα" (The male bear) and the "Η θηλυκιά/θηλυκή(you can use either one) αρκούδα" (The female bear). Most greeks would just say "Αυτή είναι μία τεράστια αρκούδα". If we had to give the gender it would probably be "Αυτή η αρκούδα είναι ένα τεράστιο θηλυκό"(This bear is an enormous female). Note: your option is not wrong it just sounds a bit strange, among those lines you can also say "Αυτή η θηλυκιά/θηλυκή αρκούδα είναι τεράστια"(This female bear is enormous
If I were lining up Greek and English words according to origin and closest meaning I would too. But here I am translating. The words huge, gigantic, enormous, ginormous are synonyms that all mean more or less the exact same thing in English and all should be accepted anywhere one is. A Greek native can help me with any Greek difference between γιγαντιαία and τεράστια but that is a separate problem.
There is no difference between the two terms other than etymology. My comment was not meant to be taken as saying that your translation is wrong and should not be accepted, I only stated another good translation match that hopefully incorporates a good mnemonic device. :) Because of the number of available alternatives, as you have pointed them out yourself, it is entirely possible that some have not been included in the answers database. So it's not that you have 'missed' something, it's rather possible that this alternative has not been included. (Also, it's interesting that you chose 'gigantic'; I don't think I've heard it much in English!)
Giant bear makes sense! I noticed your other comment about trying to use the alternate words on the exercises and it is admirable that you are trying to build your vocabulary skills. Of course, that will probably lead to a few rejected translations. But as I am sure you know, the contributors appreciate the feedback and the course is only getting better with it!
Well, if one wants to use the demonstrative pronoun "Αυτή", I think it makes more sense to translate the sentence to "This is...". The bear could be a female. of course, but then "She is an enormous bear" would mean "Είναι τεράστια αρκούδα". Personal pronouns are barely used for people (unless one wants to emphasize something), let alone the rest of the animal kingdom.