caravan gather rate?

If you trade with an TC of an teammate it gives +50%. (Effectivly the travel distance is less to get 150 gold per trip). If the map is small the distance to get 150 is smaller, on a big map the travel distance is longer. If you use the Age 4 upgrade or Zeno the distance also get smaller to get 150 gold per travel. You can measure the distance with the "Display trade preview" hotkey. Just look at the timer how long a caravans takes for a trip back and forth e.g. 30 seconds for 150 gold= 150g / 30s = 5 gold per second.

 

Edited, I was mistaken before: You don't need to hold the shortcut, you need to use the shortcut while you're choosing where to build your market. So pick your villager, chose market, then use the shortcut while your mouse is over some valid place for the market.

As Donar said the travel needed for 150 gold depend on the map size, so the gold rate is lower on bigger maps, it's hard to answer you but there are some data out there:
https://unitstats.projectceleste.com/
Some civs have the Road technology for +20% caravan speed, with only the saddle bag technology it says as gold/sec:
2.94 (small map 150m)
2.08 (average map 300m)
1.61 (huge map 500m)

Aryzel posted a google doc in the tips section with more data, it may be old, but it's unlikely anything has changed:
https://docs.google.com/spreadsheet...o61pzQyDTPILsaH6f_XTx-fXo/edit#gid=1072424742

 

Every map is different. You better figure out how to use the hotkey, the default "control + z" is working, but you should put an easier hotkey (possible only one and not a combination). You have to use a villiger or engineer, then click on market but don't set the final location. In this state you have the market selected but not yet placed. Only in this state it is possible to use the "Display Trade Preview" hotkey. You should try it a bit and it will work. If you see small green numbers then you done it right. The direction of the numbers show how much gold you get to the town center in this direction but it is possible to overlap. But usually it is easy to use because market is one of the first things to build in a quest, so not many town center will be build in this state.

 

Hover over portrait and look for "Speed"