Returning player - no way to gather high quality items and money making?

A big change that occurred since I last played is that it would seem they completely removed high quality gathering items from the game.

This is a major bummer to me because one of my most consistent ways to make money was to gather 10 high quality items of things that are Grand Company Provisioning turn-ins. Hardly any other players ever did this, and people leveling gathering professions would pay handsomely for an easy high quality turn in for easy experience.

Was there no substitute for these provisioning turn-ins? Did SE really just remove the double experience possible with these grand company provision missions for gatherers?

Did this change severely gut the money making potential of gathering overall? Is it possible for gathering to make gil for anything that isn't end-game?

You are correct, HQ gathered items are no longer a part of the game. The GC turn-in experience you gain was rebalanced around you no longer having the option of NQ vs. HQ.

I was skeptical at first, because simplifying a system usually makes it more boring. But I was wrong, gathering and fishing are actually more fun for me now that I can more quickly get the quantities I need with the new skills, and my inventory is not clogged with two of every item. (And fishing introduced several new mechanics that play off of the "large" catches that replaced HQ fish.)

Regarding the market for the GC turn-ins, I wouldn't worry. Your market is still there. There are still those who would prefer not to leave town and buy everything they need off the MB, and you can still move your stacks of 10 no problem. Every time I go through a crafting project I gather a little more than I need to craft a little more than I need, and sell off the excess. I'm always happy with how fast and for how much the gathered materials go for. They are among the fastest items to move. You can make gil reliably on items gathered from any expansion, including ARR.

The details are probably going to work out differently than what you were used to. As you said, there is no "higher-quality" stack of 10 that people are going to drop money on over a NQ set so maybe you won't get the price you did before. But the fact of the matter is that most people sell 99x stacks of materials and 10x stacks are uncommon. You will still stand out, and sell quickly. And you can get those stacks made faster than before during your gathering runs with the new Gatherer's Boon skills.

That's been my experience in any case! I'm pretty positive on the changes overall.

It was done for two reasons. Inventory bloat and the community didn't buy the high-quality materials because they felt confident they could craft a high-quality item without high-quality materials.