Quite a bit of grav effects actually from CT on, but not necessarily directly to CharStats.
LBB1 takes it on with encumbrance, the ability to carry being a straight percentage off of world size and potential variables with alternative world densities delivering different gravity. There is also an explicit gravitational effects chart for this. No movement speed effects noted though on the basic one band/25m model. STR determines encumberance, so indirectly it is adding or subtracting to STR ability, although it is not plugged into the combat mods for the STR-based melee weapons.
RUle 53 of Striker deals with the effects of planetary environments and does quite a bit in just a few paragraphs, defining many issues including gravity and world size. The former affects movement of both infantry and vehicles and drag dropping projectile round ranges, and world size deals in terrain, slope and horizon effects. The related effect of atmosphere to world size is also defined. As such STR or END is not dealt with, but the defined variable values could be used as inputs for such rules.
Rule 80 of Striker deals in integration with Traveller including skills and CharStat mods, but does not address gravitic effects directly.
The other primary set of rules with the original books and games was LBB4 SS and AHL's zero-G rules. Definitely affects movement and maintaining control, but again no gravitic enviornment direct impact on CharStats.