Other Ideas:

• It’s there, but HFWG simply missed it. The thesis does suggest that the technology involved was unlikely to reveal a roman road, and they had significant problems in accessing the data they did manage to obtain. The project was also time pressured, adversely impacted by weather and the availability of fieldwalkers.
• … but if they missed it, so did the physical excavations reported in TLAHS; excavations showed a road where it was expected, but (bar one possible result, subject to a caveat by the dig director) no evidence was found where ‘Roman Road – course of’ says the road should be.
• Perhaps it was never there, but ran somewhere else (where the evidence may or may not be discoverable).
• Perhaps it was there, but the evidence is all gone – alluvial deposition over years of flooding in the Welland valley, robbing out, ploughing out.
• Is it possible that the road, although laid out, was never developed to the extent that physical evidence would remain? After the road was laid out perhaps the physical use went two ways, leaving the gap; did the road run NW from Glooston Villa towards Leicester, while the associated alignment ran SE from Medbourne (a known Roman town) towards Corby and Huntingdon, resulting in an underutilised ‘gap’, which was never properly developed?
• Perhaps the road was built during, or immediately post conquest, but the military placed greater reliance (and further developed) Watling Street as the main SE – NW axis; see Davies 2002 cited in Turville p14. If military operations in the East Midlands were relatively brief before establishing an interim frontier zone along the line of the Fosse Way, this might account for the lack of formal development of the Gartree Road. Perhaps the army engineers pushed it through the landscape, but the campaign moved rapidly on and the road lost significance.
• I’m looking for clues after a 1500-year gap. The Glooston Villa is dated to the C4th. What if the owner of the estate continue to supply post-Roman Leicester; the ‘gap’ towards Medbourne fell into desuetude, while the road north west was kept open and remained a track in the landscape? Even a shrunken post-Roman Leicester may have been an important market, there-and-back in a long day? That might account for the differential survival of the route.