don't be too hasty in condemning the GP...
firstly, "I can't see it" from the nurse may mean just that, not, "it's not there", but, "it may be there but not sure if I'm looking in the right place".
If your Gp did have a clear view of the eardrum, and the grommet had recently fallen out, but the hole not yet sealed over, it may well look like the grommet is still there. Grommets are tiny, and the same pearly white colour as the eardrum, so it wouldn't be a huge mistake to make.
Rather than rushing back to accuse your GP of lying to you, why not ask if he/ she is sure what he saw, bearing in mind what the hospital said?