Cerebrospinal fluid is produced in the brain, I would imagine that the contrast would end up in newly produced fluid as well, but maybe that assumption is wrong.
Even assuming that the contrast could show up in newly excreted CSF (maybe, maybe not), MRI contrast elimination half lives are very short (mostly all under ~2 hrs, excepting cases like renal dysfunction), and cerebrospinal fluid doesn’t replenish particularly quickly.
Unfortunately no, that is not how the contrast agent gets around. Most Gd agents are huge and don’t leak out of the vasculature unless there is a disruption (tumour etc).
