Here's some pointers on great merchants, and how in my opinion "running a trade route" is almost always much more valuable than settling the merchant.
I'll take an example from a recent game (marathon / monarch /huge)...
I pop my 1st GM and the trade run produces 3300 gold going to Rome (adjudged the best available city at that time 300AD). Now at that time I'm massively expanding, and am falling behind techwise, with half a dozen "new" cities holding back my economy. I immediately switch to running 100% science and discover alphabet, calender and code of laws before the money runs out. Now that would be worth the trade run on it's own, but the new tech allows me to trade for half a dozen
other techs, I wouldn't have had, as I had no tech trade leverage. Therefore the GM trade route run, has directly resulted in 9 new techs.
The 2nd GM arrives in around 1100AD, and runs to Rome this time netting 5500 gold.Again, because I've a large expensive, expansive empire to maintain, I'm slightly lagging behind in tech, with nothing to trade. The cash is converted to science at 100% rate, and music, (amazingly I was first), then guilds, then banking are reserached, and these again traded for 4 or 5 other useful techs.
And its not just the sudden "tech splurge" that running a trade route facilitates, it's also all the possibilities resulting from the results that have to be taken into account. Faster COL, gave me courthouses much earlier, which saved money for every turn they were built of course. Calender allowed me to trade excess resources earlier. Banking allowed me, well banks quicker, which again is 50% extra cash for each turn they exist.
What I'm trying to show, is that there is a lot of "hidden" bonuses that a GM trade run can facilitate. It's not just 4000 gold vs 6 gold and 1 food per turn for ever, it's a whole lot more. Almost the only time I'd personally settle a GM is very late in the game, when money has become so plentiful, a large cash infusion is no longer that necessary. In virtually every other instance, cash is far too valuable, even if it's only used for massive troop upgrades.
I have to add that the above probably works best with a large empire, as it allows you to "unlock" the potential of your empire at a time when upkeep costs are stifling your economy, but hey thats the way I like to play