Agree... invitation does not equal gift, or attendance. Feel free to ignore, unless there's an RSVP notice, in which case you can RSVP no.
Not sure how I feel about grandparent showers... With showers there's such a fine line between the guest of honor wanting it, and other people really wanting to throw it. If a grandparent was going to unexpectedly be taking on primary care of a grandchild due to some unfortunate circumstance, I could see the grandparent's friends maybe getting together and saying, "Let's cheer Betty up by having a little party for her and getting her a couple of baby things." In which case, no one who isn't close enough to know the entire backstory should be invited. Or maybe, as a PP said, "any excuse for a party" with a cake and a couple of onesies as gifts, no big deal really.
But inviting a large number of people who don't even know the guest of honor, and providing a registry of expensive items only (which suggests the level of expectations the GOH has), is rather dodgy.