Hmmm...I think that your approach is the right idea. My kids are in a blended family as well and luckily, my side and their dad's side (who is now my ex) have always been even-handed with gifts. My step-daughter got the same from my family as my sons, and my son from a prior relationship got the same as his step and half siblings.
That said, my SD lived with her mom until she was 13 and during that time, she would also get gifts from her mother and her mother's family. My oldest son's father was out of the picture so he didn't get the same. Although my SD and oldest son are the same age, she got a phone before he did (from her mom) and a laptop before he did (from her mom). For her birthday and holidays, she would get gifts from me and her dad, my family, my in-laws and then get gifts from her mother and her family that our boys didn't.
Are the extra gifts coming from the boys' biological dad and his family? If so, then your situation sounds similar to mine and honestly, it wasn't something that the kids really noticed or cared about. If their mother's family (your granddaughter's step-mother's family) is being uneven in gift-giving, then that's perhaps something your son should work out with his wife because that would be hurtful in the long term and family is family. When we first blended families, I had to nudge my siblings a bit about the fact that my SD was now on the gift list too - they weren't ill-intentioned, but were a bit clueless. Perhaps you can share your observation with your son and see what he does with it.
If your granddaughter notices the difference and feels slighted, and you have the means to provide more, perhaps you can simply meet some needs outside of a gift-giving situation. For example, if the boys have electronics that they received as gifts that would be useful and appropriate for her, you could provide the funds for those and there really doesn't need to be an explanation. If anything is explained, it can simply be "X and Y already have [whatever] and Z needed one too, so grandma helped pay for it." The boys are old enough to understand that it's not necessarily fair that they get more items and I wouldn't think this would cause a problem - it wouldn't for my kids. My in-laws have provided specific things as needed over the years because they have the means to do so - they paid for after-school care at a camp for a year, pay for piano lessons for one child, provided a laptop, will pay for a specialty sports camp, etc. These are just opportunities for them to provide for the kids and cover a need and not gifts. The kids don't keep track as their needs/wants are all being met equally.