Well, there is no one or best answer. Several tactics can work, and can be best exploited by considering the disposition of your character. One approach is the subject of this thread. Oblivion gates are treasure troves for loot, especially at higher levels. If you allow them to remain open and farm them regularly, you can make a small fortune selling daedric loot, and enchanting items with sigil stones, if your character is so inclined.

Most of my characters are mages and join the mages guild first. Even if your character is not guild inclined, I suggest joining at least one early on. The best "early" perks in the game include the ability to loot..... I mean borrow equipment such as calcinators, retorts, potions, axes, swords, etc that you can sell for profit. Dungeon diving is also profitable and the rewards scale as your level increases. For my characters, alchemy is what takes them over the top. They eat all the ingredients they can find to discover the benefits. They keep a watchful eye for extremely valuable ingredients that make quality potions, and will stop to collect them as needed. They also steal/upgrade invest in the highest quality alchemy equipment (master level) as soon as it becomes available. They store all of their ingredients near the alchemy table in the wizards tower to churn out the potions in bulk for sale. Both of my Breton Sorceresses have an alchemy stat of 99. The wizards table buffs this to 114. Using master equipment, they have both cleared over 500,000 gold before level 30.

A faster and more simplistic approach if you dont like alchemy: Make friends with Tamika and the Surille Bros outside of Skingrad. Farm their graqes early and often. Not sparingly.
Use a lawnmower approach and cut each row carefully and sequentially. You should score 80-100 graqes per vineyard. Farm corn or lettuce from the other farms in the game using the same approach. Then create the infamous "restore fatigue" potion and sell them for profit. Rinse and repeat every few days. This is a good approach for characters who dislike getting their hands dirty in dungeons. My characters prefer a combination of each approach.

excellent, thank you. my character has been doing alchemy since the beggining but as a bosmer archer not inclined to magic it doesn't level up very fast. Either way I've started to sell off my potions to make some dough. i find it difficult or tedious to always find the best deal with merchants however and most of the time I simply trade with whoever is closest.

Back to the OP. When you finish the game and all the open gates close, the creatures already spawned around those sites don’t go away. They will remain around until you or some other creature kills them. So don’t fast travel back to one of them to make a few pictures without expecting a fight.