I handle this by creating a one time workflow to generate the grid and save it. After that is complete, put a 2nd workflow together that reads the new existing grid.
Something like:
Workflow #1
Molecule Reader > Protein prep > Glide Grid Generation
Workflow #2 (for docking ligands)
Molecule Reader (one for protein, one for ligands) > LigPrep (for ligands) > Glide Grid Reader > Glide docking > Molecule writer