Generally, you're only required to provide changes if you are distributing your modified program. This may have been partially addressed in GPLv3 in the form of an "application service provider" clause, but I don't recall
I doubt they even fell under the "application service provider" clause; speed-trading software is usually used completely internally by a financial firm to make its own trades.
Not necessarily, as GPL3 didn't close the so-called "ASP loophole". Since Goldman Sachs probably wasn't distributing any HFT applications, but just selling access to it or using it completely internally, they aren't required to make their modifications public. See http://stackoverflow.com/questions/2127246/difference-betwee... about the AGPL license which addresses the 'application service provider' issue.
