The concept insider trading as opposed to outsider trading is invalid. There is no such thing as outsider trading; there are only degrees of how close to the inside one is.
[1] All mutually voluntary trade between two individuals is moral. There is no reason to consider insider trading (where one party knows more than another) as immoral (unless one party defrauds the other - but that isn’t limited to insider trading), since it is by nature voluntary, uncoerced.
[2] The “economy”, like “society”, (gotta love scare quotes) is nothing more than the sum of the individual relationships among individuals. Both individuals party to insider trading benefit or they wouldn’t have volunteered to trade.
What would happen to the GDP? Well, people are now engaging in an additional form of trade: the GDP (as a placeholder for total personal wealth or something actaully meaningful) will increase.
[3] One cannot call insider trading as such fraud; it is only fraud if one of the parties deceives the other.
GreedyCapitalist: Want to buy my company?
Mordecai: Only if it’s not on the verge of bankruptcy.
GreedyCapitalist: [lies] It’s not.
Mordecai: Ok, I’ll sign.
[4] I can’t answer the fourth question, it’s asking me to hypothesize what the universe would be like if gravity were repellent, not attractive. Whether I knew the answer or not, the answer would not make the slightest difference: gravity is and will always be attractive. If something that weren’t the case were, what would be?
To cripple the economic system means to initiate force against the individuals who engage in trade to force them not to trade - that is the moral basis to ban crippling the economic system.
If people trusted each other less, perhaps they would trade less; but, how does the degree of difference of knowledge lead people to a difference in degree of trust?