Agree with OP
You gain a substantial amount of gold from a golden age. If you're a.)Darius, B.)acquire chicken pizza, or c.) have finished Freedom (and these bonuses stack), it's quite conceivable that you'd gain enough gold from a golden age to not only equal the amount of gold a GM would grant, but have enough left over to buy the influence that the GM would grant. As if that wasn't enough, don't forget that every single city in your empire gets a 20% bonus to both culture and production. There's no comparison; GM<GA. GSs and GEs are also more effective in my opinion, but I'll digress as those are more arguable(but really?). GProphets (along with Ggenerals/Gadmirals) develop along a different path, so they don't really compete directly.
What really bothers me about the merchant class is the merchant base yield as well as the customs house base yield. I can see engineers making only 2 hammers vs. arists/scientists having a base yield of 3, but merchants getting only 2 gold is questionable. Same as customs house getting 4 gold whereas academies have DOUBLE the yield in beakers, and landmarks/holy sites have 150% the yield in their respective areas. I guess the point is how does 1 gold compare to 1 culture, 1 beaker, 1 faith or 1 hammer. I feel that 1 gold is slightly less than 1 of each of the other. If this is NOT the case, then dedicating the hammers or gold to produce any building in any city is a questionable move because as buildings cost maintainance, you're basically making a tradeoff between gold surplus and beaker/hammer/culture/faith surplus. This is most directly noticeable with faith as it's a 1:1 ratio with both buildings: a shrine trades 1 surplus gold(paid in maintenance) for 1 faith, and a temple trades 2 surplus gold/turn(2 maintenance) for 2 faith/turn.