General synonyms:
* Brave
* Courageous
* Chivalrous
* Noble
* Valiant
* Heroic
* Bold
* Daring
* Magnanimous
* Generous
Synonyms emphasizing courtesy and politeness:
* Courteous
* Polite
* Attentive
* Charming
* Gracious
* Well-mannered
Synonyms emphasizing romantic gestures:
* Romantic
* Suave
* Charming
* Debonair
* Handsome
The best synonym will depend on the specific context in which you are using the word "gallant."