var query = from g in db.Galleries
join m in db.Media on g.GalleryID equals m.GalleryID into gm
where g.GalleryID == 100
select new { g.GalleryTitle, Media = gm };
Property Media
will contain list of joined media entities. Also if you have navigation property defined in Gallery entity, then:
var gallery = db.Galleries.Include("Media")
.FirstOrDefault(g => g.GalleryID == 100);