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I completely agree with the accepted answer of @David Richerby from personal experience.

As a heavier rider (~22 stone) for many years, I also found with a couple of different bicycles that the rear wheel tended to come out of true, especially when hitting bumps and potholes.

My solution was to replace the rear wheel with a touring wheel with more spokes, and I can ride over speed bumps etc. with less worry, although I do ride them standing on the pedals to reduce the force through the seatpost.

I also learned how to true a wheel reasonably, and to keep it trued and correctly tensioned, because shocks have less effect on a properly balanced wheel. It needs mostly patience and care, and something simple to act as a reference marker.

Spokes getting loose is not a laughing matter. The more spokes get loose or break, the more uneven the load is distributed to the spokes, and the more likely other spokes are to get loose or break. Accumulate enough failed spokes, and your wheel fails. And failed spokes after only 24km means that something is very wrong with the wheel's built.

As such, your wheel definitely needs to be rebuilt immediately.

When the wheel is rebuilt, it needs to be rebuilt such that no spoke can ever loose tension when you use it. Most importantly, that means that the spokes must have even tension, but also that their tension must be high enough. I'd recommend giving the wheel to a professional builder, and to tell them that the wheel is supposed to carry you. Hopefully, they'll be able to produce a built of high enough quality to avoid repetition of the failure.

What you can do to reduce the likelihood of failure, is to avoid accelerating too hard: The force you can put on the chain is much higher than the force a 70kg person can put on the chain. And that force on the chain is directly proportional to the torque that your spokes need to deliver to the rim. If you put your bike in first gear and then stand on your pedal to accelerate, you'll pretty much kill any rear wheel. Just try to take acceleration a bit more slowly until you have lost weight.

For your weight, you either need to find reliable wheel builder who will make wheels for you, or you need to become one. Possibly both in that order.

Wheel will be most important part of your bicycle and you should strive to put best components in it, and be sure it is built with your weight in mind.

Unwinding of spokes is almost certainly result of not enough of tension on them for your weight. To be able to put enough tension, you need as many spokes as you can, and very strong rim. It should have no less than 36 spokes, heavy duty rim like rigida sputnik or andre, and double butted spokes.

It is possible to re-tension loose spokes at home, using a spoke key and the frame of the bicycle as a truing stand. In your case I would not do so.

If the wheel started to fail so quickly from new, I think it would fail again even faster with a home repair. In the worse case the wheel could fail catastrophically injuring you and damaging the bike. In the best case you get stranded.

When you take the bike in for service, the repair shop will hopefully be able to re-build the wheel. They may be able to build it to a higher strength than it was originally by increasing the spoke tension, but it may fail again of course. If the wheel can't be re-built or fails again, you can consider getting a stronger replacement wheel with a higher spoke count and heavier gauge spokes.

Loose spokes and buckling wheels on a new bike are definitely not normal.

That could be incorrect assembly but may also be a result of the weight of the rider and not getting the right advice:

The design specifications in the GT tech manual book list "330 lbs / 150 kg" as the maximum weight limit for that model. Although not explicitly mentioned for the Avalanche I assume that is the limit for the combined weight of the rider and all gear.

I would suggest that if visited the shop and they helped you choose the bike then they should exchange the bike for something more appropriate.
Clearly the bike is not suitable otherwise the issue you raised would not have occurred.

Upon getting home I have noticed that some of the spokes were loose. Is this normal and if not is it due to the weight the bike is being asked to carry? Will a home fix be enough to mend the issue?

In my experience, loose spokes are not typical but can happen. Any decent bump (especially for a big guy like I am) can remove pressure from a spoke and if it is twisted, it will unscrew a bit. But twisted spokes imply that the wheel was not built properly.

When building a wheel, I was taught to over-tighten the spokes and then back them off specifically to reduce spoke twisting. You want the spokes to be under tension but not twisted. See this answer for more details: https://bicycles.stackexchange.com/a/29696. I also was trained to grab the parallel spokes and squeeze them to help untwist the spokes while truing a tire. This page explains the squeezing well.

I also know some wheel builders who use Loctite solution on the spokes to hold them better although I've never found this to be necessary.