The basic rules of etiquette of a gentleman never go out of fashion: open the door, be punctual, offer her coat, stand when a woman enters the room, and bring the chair.
If you want to be a gentleman, wear clothes that fit you, throw the flashy clothes and avoid fads that make you look ridiculous (e.g. baseball caps tilted to one side). Wear solid and simple colors like black, gray and brown. No need to wear shirts with graphics, neon green shorts or terribly big watches. Keep it simple and you’ll look elegant and gentlemanly.
However, those small details are only half of what a true gentleman really is. The other half consists of intellect and education.