Which bike?
 

I have a GT Aggressor Pro. And pro it is... at having issues. I am looking to upgrade to something from a better brand like trek or diamondback. I have no idea at all what to get. I have a very small budget of $800 and live out in a very small town where everything is on a hill. Anyways, if I don’t have a spot to use a mountain bike as it should be used (I mainly ride on road with a little gravel) what should I upgrade to? I don’t want a used bike though.

A bad time many places to even find a bike shop that isn't sold out of bikes. May be best to call around and see if your LBSs are open or have anything in stock and test ride if they do.
Otherwise changing the bike you have over to street type tires make excellent urban bikes

Facebook marketplace and Craigslist. Lost of options for $800 since the market is hot right now.

A CX or Gravel bike might be a good All-road since that’s what they are sort of designed for. You can get them geared more like a road bike or like a mountain bike.

I can give you my view - I would buy an old road bike and slap a brand new groupset on it. Neo retro is the way to go for me, there are many excellent old road bikes around and steel...is real!

You will probably fit in your budget if you also sell your current bike or sell the old parts from your new one. I see Shimano 105 R7000 groupset for 600$ in US websites.

I find such a project worth it because you quickly learn everything about bike mechanics, you are involved in it personally and as a result its not just a random bike you picked up at the store.

http://www.bikesdirect.com/products/...-roadbikes.htm

Not a Trek or Specialized, but 50/34 chainrings with 11-34 cassette will get you up some serious hills, and Gatorskins will likely survive a bit of gravel.

Honestly the bike you have looks pretty good. If it is reasonably comfortable and works well, I would try installing some narrower semi-slick tires first as suggested in another post.

While I love my entry level Giant Sedona comfort bike, Low end, which might be what $800 will get even from more popular brands may not be a lot better then what you have now. Mostly different. Which can be a good thing.

Hybrid bikes like Giant's Roam or Escape might be a good bikes to check out if your unsatisfied with your bike after trying it out with quicker tires. These adventure bikes work well on paved roads, gravel, and many trails. I believe they have low enough gears for pretty steep hills and should be within your budget. Trek and Specialized are other brands I trust that probably have similar bikes to Giant's Roam and Escape.