I only have a couple of Heavy Gear books but they are some mentions of landships existing. I haven't found any stats for them. It's possible they are in a more miniatures focused product but I was wondering if there any books that specifically detail them and their uses? Books with stats would be a plus!
On DreamPod 9's Downloads Page you can find some record sheets for Landships under HG RPG & Tactical -> Heavy Gear Tactical. Additionally, it appears that if you use the Vehicle Construction Rules (Vehicle Construction System, VCS Worksheets) you can craft a Landship by creating the largest scale of ship.