well gambling has a clear definition. it is something that you can not predict at all. it is 100% pure chance. for example when you roll a dice or flip a coin there is no way to know what the result of it will be. meaning pure chance.
but when it comes to trading and prices, it is hard to predict but it is not 100% unclear. you can expect certain scenarios to happen with different levels of certainty based on the market situation. and that is a big difference that makes it distinguished from gambling.
If we presume that the absolute majority of traders are losing in the long run, then, to me, this pretty much counts as gambling.
why would you make that assumption? based on what data?
even if we assume it is true, that still doesn't make "trading" like gambling. instead it makes "traders" as gamblers. and there is a big difference there.
When you flip a coin and you don't know anything about your chances, you can also build sophisticated theories why, for example, you should get heads more often than tails. But it doesn't matter, does it? Given that there is a sort of house edge in trading too, we should unmistakably come to the conclusion outlined in OP.
first of all making theories doesn't mean these theories are correct. and when it comes to gambling such as flipping a coin you will never get more heads and tails. every flip has an equal chance of going either way no matter what happened before.
and that is one of the differences. for example in trading if there was a big rise (ie what happened before) then you can expect a drop or correction (you can predict heads or tails since one has a much higher chance of happening now)
secondly house edge means the house or the thingy you are betting against has a better chance of winning than you. there is no such thin in trading, and you are not trading against one entity. you are trading against a lot of others based on the market movements.
if you can go with the flow you win.