I suspect it's difficult to get grenades to feel right because human characters are modeled on a different scale than level geometry. Different scales are used in order to cope with the rigid player collision boxes that are used when moving around the level; if characters were modeled to the same scale as the environment then players would constantly be getting stuck on small obstacles and in doorways.
So either the gravity will feel wrong relative to level geometry or it will feel wrong relative to the character model sizes.
FYI, in HL2 I believe the gravity is roughly 50 ft/s relative to character models or 37.5 ft/s relative to level geometry. That's higher than "real" gravity in both cases, but high gravity values are pretty common in FPS games.
Also consider that the grenades themselves are usually modeled larger than life in order to make them more visible at small screen resolutions. The larger grenade models will visually give the impression of lower gravity, as your mind is subconsciously assuming the wrong size for the grenade.