Gadolinium seems like one of those things that won't even be allowed in most circumstances in a few more years. It has been used for decades and conventional wisdom was, if your kidneys are fine it all leaves the body in 24 hours. Then just a few years ago we learn this is false and that deposits can be found years later in the brain.
The effect may be highly lagged or small, but have to believe it isn't healthy.
No, not healthy at all. There is a condition known as Gadolinium Deposition Disease (GDD) that some people are developing from the retention. The mechanics are not fully understood, and there is a multi-billion dollar industry preventing (or at least not encouraging) any serious inquiry into the matter, but my understanding is that if your body views gadolinium as a threat (similar to an allergy), it will mount an immune response against it. However immune responses are futile against heavy metals. So think of it as a permanent, never-ending allergic reaction to an element that refuses to leave your body.
I thought the issue with Gadolinium is with repeated doses. The occasional one of dose is not retained to any significant amount in bones and the brain.
This is quite surprising seeing how widespread it is, searches suggest its used in millions in the US alone from any age.
The effect may be highly lagged or small, but have to believe it isn't healthy.