I think it's a combination of things. First off, 3½ gallon would yield a little less than 1Lbs Gouda so your brining is indeed strong but I don't think that's enough to kill the cheese. Many times over-brining causes a thick hard rind to form and one may think that the cheese is very hard but underneath when it is aged you will be shocked to find a soft elastic body with the right moisture. The tough dense rind had protected the cheese from over-drying. You won't be able to judge this in the early days of cheese because the enzymes have not gotten to work yet.
Likewise, you can't judge a cheese at all at the tender age of 10 days. Nothing has happened yet. The curd is still knotting. The enzymes have just began breaking down the proteins (which gives the cheese its soft texture but takes about 2-3 months minimum in a Gouda). You won't find any sharpness in the cheese because lipolysis had not taken place yet (the breakdown of fatty acids which give cheese its sharpness). The starter bacteria is still alive and doing its work and the initial aroma had disappeared and has not been replaced yet with the aroma of the fine aged cheese. Gasses and yeasts have not yet been developed, and acidity is still not in its final place - really, there is no flavor, texture or aroma, nothing to judge cheese quality by at day 10.
It seems to me that your acidification schedule got messed up somehow. Somewhere between heating up the milk and cutting the curd you have developed too much acidity (waited too long? Used old milk?). Milk quality is a serious contributing factor. While you don't need to go as far as raw milk, you need to at least use non-homogenized milk for reasonable results. If the Walmart milk was ultra pasteurized than you will have issues too because the proteins might have been dented already and there are not enough enzymes or native bacterium. Moreover, Walmart milk is likely from cows who are not local and do not graze (cheap corn, grain and silage feed). That can result in poor aging results. If the cows have been treated with antibiotics, the milk may resist your bacterial cultures and some of the long-term rennet activity. They say cheese starts with milk and is made in the vat... Use the best milk for cheesemaking and follow the vat procedure tightly.
One final note; I really find Ricky Carrol's recipes to be inaccurate, missing steps and not very fine. I too would suggest using 200 Easy Cheese Recipes. Really nice collection of useful and accurate recipes of large varieties of cheese.
