I only have about 7k gold at the moment and need mare gold to unlock the third party slot and buy a new stable tab... but it's hard for me to earn any. I do missions often but I only usually earn 1k or less a day from that. Any help is appreciated!

Are you. Sure you're doing Missions? I think you may have gotten them confused with Quests

Looking at your profile:
Battle Record: 302-0 (100.00%)
Mission Record: 0-0 (0.00%)
Gold Earned: 27,514
Scales Earned: 27
Quests Completed: 14

This is the quest board
While
this is the mission barracks

You can increase the total number of mission slots (number of missions you can take at one time) to six (though I believe you unlock some of those at Saerielle) here.
The first available missions are the Easy ones, which last an hour and can be done by pretty much any pet. So from the moment you send them on the mission, you can go and anything else, then come back an hour later and go here to Complete the mission
(Text colours for completion:
Red = Failure, no reward
Yellow = poor, no mission satchel, and I think half the base rewards?
Black = satisfactory, one mission reward satchel, base rewards
Green = exceptional, two satchels, double base reward)

With missions, you send pets to do missions, and people usually have dedicated mission pets with all their proficiency points and equipment oriented to one stat (example, our Very Hard/Lv50 Mission Dexterity pet)

There is a guide to missions and mission pets here

The base reward for a satisfactory completion of an easy Esior mission is 200 gold, 10 faction points, some exp for the pet, and one lv0 mission satchel, which generally contains either low level equipment, ~1000 gold, or a small number of a healing item

Oh.... Oops....

Is there any other ways to earn gold other than missions?

Missions are by far the easiest, but there is a guide to money making methods here
(We just know the most about missions because that's where the vast majority of ours comes from, being quick, easy, and requiring no effort on our part)

Yes.In addition to selling gear you can't wear and doing quests:

If you somehow have something other people want, and are willing to pay well for- you might have gotten something like that in the free lucky pot play, or you might not- you can sell it to them in the broker. Tragically, there is no "how to sell stuff in the broker" guide, but that's what the broker item button lets you do. If it sells, the broker will claim 5% of the item's price for themselves, though. (If it doesn't sell, you'll suffer no losses. the item will be sent back to you eventually.)

Once you get strong enough, doing dungeons is pretty good for money, though not perfect.

It's just missions give more gold-for-effort than most other things except selling stuff to other people (who get most of their gold from missions.) If I manage to complete 8 12-hour missions in a day with relatively suitable lv. 60 pets (some might be availible) and decent gear, I make between 48k and 57k gold, not counting the mission reward. You might want to look into the 12 hour one in viridian meadows. I'd think that one would be available, by now.

EDIT: I'm going to send some gear to you. If it'd be an improvement on your pets, put it on them. If not, sell it. I strongly recommend you do equip it if it's an improvement, though, rather than sell it. Sadly, I don't currently have any more specific stuff than disciple.

Thank you! I'll try to send some pets on missions this time...