I am not a experienced gambler, I just gamble in casino, poker and sports based gambling, so I have small amount of knowledge but definitely not a experienced one and not even near to be an expert.
And mostly it is happened in sportsbetting or poker as both believed as the profitable and reliable games.
We also frequently see a professional poker player that recognized widely in the world , no doubt they are the real expert.
I agree on what you say that the thing which makes a gambler being expert is on the way he control himself on playing gambling and not getting addicted into it.
Aside from that being an expert simply means to be able to know a mild difference in emotion of their opponent. Aside from that experts especially in sports betting are knowledgeable of every aspect that may change the course of the game. They can tell whether each player is in sycn with others or they have an insight of seeing the variable things that will affect the game. You can be expert and addicted to the game at the same time.
Those person whose addicted with gambling is the expert of it, and more knowledgeable of every aspects of gambling bets. As experience lead them to more knowledge I think that made them confident on particular decisions, specially the betting strategies they had. But it doesn't guarantee them an assurance to win even though they're skillful enough, because gambling is unpredictable all the time. Because of their aggressiveness for winning, they might lose more amounts rather than low level skills gambler.