Giant bikes are good values. Giant is actually one of the industry's big frame makers, Their frames are used by other bike companies. That allows Giant to basically sell you a bike frame at a reduced cost on their complete bikes compared to other companies. As long as the bike has the specs you want, I wouldn't have any problem with Giant. Have owned a couple.

The actual frame weight between aluminum and steel is actually very small. It's mostly a matter of ride quality between the two. Carbon is smoother, but a well-designed aluminum frame can also make for a very nice ride and at a much nicer price. Trek makes some especially nice aluminum frames. Some of the best I've ridden.

P.S. Don't forget steel. It has a classic, wonderful feel all it's own. It's my first choice for long days on a bike or any kind of riding, for that matter. It's heavier, but that brings up another point. Don't get too hung up on overall bike weight. It's where the weight is on the bike that counts. Number one on a bike is the wheels.

Wheel sets often get overlooked when checking a model's specs, but that's a mistake. They really make a difference. Those higher end wheel sets on a top of the line bike are a big part of why those bikes are so expensive. 105 spec'd bikes usually have good durable wheel sets, but they're not the lightest, though they are usually very durable and durability is important, too, especially on a gravel bike.