Gem to gold
From what I’ve read, no. There’s the difference in prices between buying and selling gems that makes it unprofitable to do it quickly. At best someone would have to hold on to gems until they rose in price high enough to both cover the fees and to make a profit.
http://wiki.guildwars2.com/wiki/Gem
“There is a 15% transaction fee whether trading gems for gold or vice versa. For example, exchanging 1 gold gives 85 silver worth of gems while reselling those gems returns only around 72s.25c , a net loss of roughly 28%.”
No. Anyway you can’t really flip because flipping relies on players not willing or caring to maximize their earnings or minimize their spending thus leaving coin on the table. The exchange is the only buyer and seller and it makes sure it gets the most for it’s Gems and pay least when buying Gems.
As Astral points out, there’s a 28% loss buying gems and selling them back right away. You would have to wait until the exchange rate rise about 40% just to break even and that can take multiple months for that to happen (or a major Gem Shop sale driving the rates up can shorten that time).
RIP City of Heroes