First of all I'd say that sending retainers out on missions is not the best way to make a huge amount of gil, but lets look at the options that you can use:

1) Check the items your retainers can bring back during their Hunting/Mining/Botany/Fishing Exploration and compare that with what sells for an acceptable price on your world. I'd expect that only a couple of items from the "Hunting" (DoW/M) that are used in either Ishgard crafting or high level recipes will sell well. So I'd take a look at the last two pages of Hunting explorations and compare that with your worlds marketboard. Soil from Mining Explorations, aswell as a couple of seeds from Botany (specially those that are used in furniture recipes) might have a chance to sell well aswell.

2) Use your retainers to get yourself materials you need to turn them into items that sell better and/or for more gil. You could sent your Hunting retainer out to get you the ingredients for level 80 raid food or crafting teas. If you have access to a garden, you can sent your Mining retainer out and just use the soil yourself to crossbreed stuff or grow plants that sell well.

3) There are two ways which rely on luck, but can yield some money:
(a) sent your retainers out for the 18-hour Field Exploration, picking the ones that can yield minions. Before you do that, you should check wether or not the minions actually sell for enough gil for you to warrant this. Here is a guide that includes all minions that come from ventures: https://www.thegamer.com/final-fantasy-14-retainer-ventures-minions-guide/ - crosscheck this with the marketboard and see for yourself if any of it is worth it.
(b) sent your retainers out for quick ventures and hope that they bring back something rare and useful. This has the neat side effect that they'll often bring back useless dungeon gear that you can turn in for GC-seals, which pay for your retainer ventures (or some mats, that may or may not sell well... most probably dont any more).

Gear makes a difference up to a certain point - for DoL the gathering stat is important, anything else doesnt matter. For DoW/M the ilvl is important, anything else doesnt matter. Or, in other words: You dont need the absolute best gear and HQ and melds only make a difference for DoL when its gathering materia melded.
The gathering stat or the ilvl decides wether or not your retainer has access to certain ventures (mainly the 18-hour Field explorations, the higher the better the gear needs to be), how long ventures take and how much they yield (the "Targeted" ventures, Hunting, Mining etc.) and wether or not they can find rare stuff during Quick Ventures. For the last one at least the game will give you a message after each completed venture with something along the lines of "Your retainers gear is good enough to find rare items" if your gear is good enough or something like "If you improve the gear of your retainer you can get rare items" if it isnt.

All in all I'd say that retainer ventures are best used to get you some materials you need for crafting that are otherwise time consuming to obtain. I wouldnt rely on ventures for a steady income - the ones that have the potential to pay well also include a lot of RNG.