Normalised Graded Pace is an adjustment of actual pace considering elevation changes. In other words NGP is designed to be the equivalent pace if running at the same level of effort on flat terrain. Consequently, NGP will be slower than actual pace when running downhill and faster when running uphill.
The research to evidence such calculations are relatively sparse, and as one can imagine generic, which can often lead to questionable figures. Particularly at extremes of grades, or over very undulating terrain.
References & Further Reading
Staab JS, Agnew JW, Siconolfi SF. Metabolic and performance responses to uphill and downhill running in distance runners. Med Sci Sports Exerc. 1992 Jan;24(1):124-7. PMID: 1548986. Minetti AE, Moia C, Roi GS, Susta D, Ferretti G. Energy cost of walking and running at extreme uphill and downhill slopes. J Appl Physiol (1985). 2002 Sep;93(3):1039-46. doi: 10.1152/japplphysiol.01177.2001. PMID: 12183501. Vernillo G, Giandolini M, Edwards WB, Morin JB, Samozino P, Horvais N, Millet GY. Biomechanics and Physiology of Uphill and Downhill Running. Sports Med. 2017 Apr;47(4):615-629. doi: 10.1007/s40279-016-0605-y. PMID: 27501719. Khassetarash A, Vernillo G, Martinez A, Baggaley M, Giandolini M, Horvais N, Millet GY, Edwards WB. Biomechanics of graded running: Part II-Joint kinematics and kinetics. Scand J Med Sci Sports. 2020 Sep;30(9):1642-1654. doi: 10.1111/sms.13735. Epub 2020 Jun 17. PMID: 32485036.