Gremlik's initial plan had been to steal Erlik's research and to take his place as the new ruler of Laruel. The first part had succeeded, but he had never expected that the Draugr would parlay with the Sapling instead of dominating it.
The need to obtain the Sapling's consent made it impossible for Gremlik to take Erlik's place, and with it his chance to Awaken. On top of that, even if the Grendel managed to escape from the city, he wouldn't be able to use the stolen data.
Finding another Sapling and persuading it to share its knowledge was possible, but if Leannan had really unraveled Erlik's plot, then the other city-states would lock the undead out to make sure that Laruel's situation didn't happen again.
