General antonyms:
* Unfriendly
* Hostile
* Unpleasant
* Unkind
* Disagreeable
* Difficult
* Cold
* Aloof
* Unwelcoming
Antonyms focusing on specific aspects of amiability:
* Cantankerous: If you want to emphasize the opposite of being good-natured and easy to get along with.
* Sullen: If you want to emphasize the opposite of being cheerful and pleasant.
* Aggressive: If you want to emphasize the opposite of being gentle and agreeable.
* Rude: If you want to emphasize the opposite of being polite and courteous.
The best antonym will depend on the specific context you're using it in.