First, let me explain what a 'Gimmick Character' is, for my thoughts.
A 'Gimmick Character' is a character that an overblown schtick.
Let us take the 'Sci-Fi Gunslinger' as an example.
Character is built around the Gunslinger look and feel made popular by Hollywood westerns such as Tombstone. He is a sure shot with a pistol, and almost as good with his rifle. Can do the bareknuckles fighting and the gambling, usually playing poker or whatever card game fits the setting.
He is cordial to the ladies, speaks in strange vernaculars and has a rough and tumble exterior.
"So what is wrong with that?" you ask.
It depends on how far you take that character.
If you focus everything around 1 thing that defines the character, it becomes a 'Gimmick.'
He walks bowlegged, chews 'tobacky,' constantly talks about horses, checks his pocketwatch every 30 seconds, you have the need to describe his spurs at all times, has a HUGE belt buckle (which, fyi, they really did not have), kills people over the slightest insult towards his horse or his hat, is always drinking 'rotgut,' and is always asking for 'rotgut' in a campaign setting where it will obviously not be available.
That is the way to play that character as a 'Gimmick.'
Want to play the cowboy and not be a 'Gimmick' character?
Easy, describe your character, once for each new player to the group. Focus on what makes the character who he is, not who he looks like. In other words, don't over-do it.
I played the cowboy above, and yes, he had a 'gimmick.' But it was downplayed. For the most part, he was just an offworlder trying to make good in his job. He tipped his hat to the ladies, did not back down from a fight, and 'shot straight.'
But I did not play up the gimmick to the point of making the entire focus of the character, "Hey look at me, I'm a COWBOY!" I tried to leave it all in the back of the story.
Because the story was about the group of characters, not just mine.
Indiana Jones had a schtick, his hat. Yes he would go back for it.
Indiana Jones as a gimmick character would name his hat, talk to it, have his hat define who he was.
Want to be the 'loner' - read a Choose Your Own Adventure book
Want to be the 'Emotionless one' - those are called Sociopaths, you are the bad guy
Want to be the 'Volitile Angry Guy' - those are Psychopaths, again, that makes you the bad guy
Don't be a gimmick, be a character.
Work with the team, the game is not just about you.
Get stuff done.