Lactic bacteria continues to multiply rapidly if your cheese and room are too warm in these early stages. With some cheese the formula allows that because they have a different process, pH curve or aging style. Gouda process requires cooler room (think Holland weather vs. Sicilian weather, right?) It is especially true with cheese like Gouda, where you are not trying to bloom molds and yeasts on the rind.
Brine: should ALWAYS be cool (unless you have a recipe that says otherwise). I say 60°F +/-5°F would work great. You don't want it too cold because the cheese contracts and stiffens and water density is higher so the osmosis activity of the salt is reduced too much and it may over-salt the exterior before it has a chance to get to the heart of the cheese. You don't want it too warm because it will accelerate the lactic bacteria, soften the rind instead of stiffening it, and it may salt too quickly so it will end up over-salted.
Room: air condition it or go to a cooler room if you can, even press it right inside your cave if possible. Think 65°F or so.
Culture: Don't use that mystery C101 culture. It's not worth it throwing away gallons of expensive milk and months of aging and cave real estate only to be disheartened just because you have some proprietary unpredictable culture. Use a good culture from one of the major culture houses, such as Danisco Choozit KAZU which is probably used in much of the Gouda you buy anyway... for Gouda you need a farmstead culture (mixed meso+thermo) so also commons cultures like MA4000 series (which you can use for anything from Camembert to Tomme to Gouda to Cheddar) can work too. It doesn't give you that traditional dutch flavor/texture though like the Kazu.
Milk: That could be your issue. Supermarket milk is not worth the effort. Especially with a cheese like gouda that has no rind and little salt and it needs to speak volumes for the milk quality. Typically it is made with corn/grain/silage fed cows (industrial cattle junk food) which is laden with antibiotics galore (which are fighting your probiotic lactic bacteria). Moreover, homogenization breaks open the fat globules and basically kill the milk. You have small yield, weird acidification schedule and results that are chalky and brittle. These are also often HTST-pasteurized so they are basically dead - enzymes, minerals, lactic bacteria, amino acids etc. It's just really not proper for cheesemaking beyond fresh spreadable cheeses or simple casual yogurt/labaneh.
