I went through this exact scenario (kirkland food, deworming, bland diet and all) with my doberman. He only stopped having gas/diarrhea once we got on the bland diet. After trying one more time with the Costco food (which my other two dogs do great on), and going right back to gas/diarrhea, I relented and tried a whole new method of feeding.
We switched from the bland diet to Grandma Lucy's dehydrated, all human-grade food, and his problems went away overnight. It's not cheap, but it was the most cost effective, non-kibble, easy-to-digest, limited ingredient but complete diet solution I could come up with. It's $68 with my subscribe and save Amazon account for 10lbs that makes 50lbs of food, so hopefully will last us about 6-8 weeks. The consistency is that of mashed potato, but my dog LOVES it, and we've had no more problems since starting it just a week ago. Totally worth not coming home to diarrhea puddles every other day, and no longer suffering through room-clearing gas.
Not saying this is the only way to go, but if you're finding kibbles to not be working and you've spoken with your vet, I'd try this before jumping to anything more invasive or complicated. Some dogs just need more basic diets, I think, and wet food or dehydrated food is often easier for them to handle. Good luck to you and your pup!
Food is a hot topic on here, but I'll try to offer some suggestions to help.
First off, if this came out of no-where, you might double check with your vet to make sure it's the food, and not a parasite. If your dog frequents any dog parks or anything, these can be fairly easy to pick up.
Once that's clarified, do a bland diet (rice and chicken or scrambled egg) for a few days to see if there's improvement. Then I'd try to reintroduce the kibble over the course of 5-7 days. However, if you just want to find a new brand of food due to personal reasons, that's okay too. A few things to consider: if he was doing purina pro-plan, what flavor was he using? Maybe consider something different? For instance, I've seen many dogs have a harder time with digesting beef than chicken or turkey. My greyhounds have always done well with Costco's Nature's Domain food, which is made at the same factory as Taste of the Wild (if you don't have a membership). It costs around $30 for a 35lb bag at Costco, and is IMO the most cost effective, higher quality kibble on the market. Another idea might be to stick with "sensitive stomach" blends (since you mentioned this as a concern) and look into kibbles with more basic, limited ingredients that are still nutritionally complete. Natural Balance has a few, and my female hound did well on their chicken and oat recipe when we didn't have a Costco near us for a period of time. More recently, my Doberman was having a rough time with kibbles, even the Costco blend my other two dogs have always done well on. After a vet visit, bland diet, precautionary deworming and trying it again, we decided it was the food he was struggling with, and, being tired of frequent diarrhea puddles in the house, I otped to try something more drastic for him, and we gave Grandma Lucy's dehydrated, human-grade dog food a try. The ingredients are all very simple, but still nutritionally complete and is produced by a small company in California with 87% of their ingredients sourced from the U.S. It costs me $68 for what makes 50lbs of food with my subscribe and save option, and honestly, my dobie's issues went away overnight. No more gas, no more extreme thirsty, no more itchy dry skin, no more diarrhea. The stuff has the consistency of mashed potatoes, and my dog LOVES it. Not saying this is the only way to go, but I do think some dogs are more sensitive than others and benefit from a more basic diet if possible, and it might be worth looking into alternatives that a) He does well with, b) You feel good about too. Whatever you chose, go slow, and do consult with your vet to rule out any other possibilities. Good luck to you and your lurcher!