Gilbert is a very cute guinea pig.
I used to have a guinea pig that had stones, and the vet recommended vitamin C chewables. I forget which brand and dosage at this time. It took trying a few brands to find one he would eat, but then it cleared up his stones like magic after a few days. It was amazing.
Also, we had to stop feeding certain veggies. I think kale was one of them. I think oxalic acid was the culprit ... it's been a while, but it worked for us, so it's worth looking into.
Just wanted to help. :-)