You can try to re-enable Mass Storage Mode using QuickCure3 which is available towards the base of this thread's first post. It's universal and doesn't involve the actual device firmware at all, but it doesn't always work like Cure3 made from original fw does consistently. It's safe to try it because as it shouldn't make things any worse even if it doesn't work to restore MSM to allow file removal. However, if successful in removing the offending GPX files, you might still need to load the original (non-cure) firmware somehow afterwards if the device remains stuck on the start-up screen after you do. It should just boot normally though once you remove the known-bad files.