Some possibilities, depending on the nuance you want to capture, could include:
* Κοιλάδα (Koilada): This is the Greek word for "valley." While not a name itself, it could inspire a name. You could potentially use a derivative or a name with similar sounds.
* Names associated with mountains or nature: Greek mythology and geography are rich with names relating to mountains and nature. You could choose a name with a similar feeling to Glenn, like a name associated with a specific mountain or natural feature. However, this would be more of a thematic equivalent rather than a direct translation.
Ultimately, there's no perfect translation. The best approach would be to choose a Greek name that evokes a similar feeling or has a similar sound to Glenn, rather than a direct linguistic equivalent.