Editing a Gallery in LR4

Hi all

I've used LR4 to create a couple of galleries but would like to be able to add to them in the future.

I've tried to create a new one but I can't seem to use the pictures that have been imported once already - they stay greyed out in the import module.

I've also had a good look for an option to pull the gallery back in but I can't find that either - is it a case of publish once and that's it or is there a way to make changes?

Thanks for any help or advice.