"D" and "L" aren’t gear settings, they are regen level settings. You don’t "drive" in Low, you "slow down" in Low... D and L are settings for determining how aggressively you want your Volt to reduce your momentum whenever you take your foot off the accelerator... do you want the car to "coast" in D (relatively light regen) or do you want to quickly reduce your Volt’s momentum by engaging the stronger L-level regen each time you take your foot off the accelerator?
Regenerative braking is an alternative to friction braking in a Volt, and it doesn’t matter if you are driving in Electric Mode using battery power or in Extended Range Mode using gas... once regen puts power into the battery, however, the distance you drive on that regen battery power will be classified under the driving mode you were in when you created it... driving downhill in Hold Mode or with a fully depleted battery will give you regen-battery-powered Gas Miles once you reach the bottom of the hill and start using that regen. If you drive down the same hill in Electric Mode, the regen-battery-powered miles will be recorded as Electric Miles.
On the highway you may, of course, simply "ease up" on the throttle without fully invoking D or L (for example, you just accelerated to pass someone and have now moved back into the driving lane and are slowing back down to that speed)... with a Gen 1 Volt, easing up on the throttle lowers the amount of fuel supplied to the motor to the amount needed to maintain the slower speed, and the car will slow down to that speed with no regen created. With a Gen 2 and its two planetary gear system, when you ease off the throttle, MGB may apply negative torque (i.e., regen) while propulsion torque is still being applied to the wheels by MGA and/or the engine. GM engineers have apparently determined that under certain Gen 2 driving conditions, using regen to slow the car while continuing to provide propulsion torque to the wheels sufficient to keep the car moving at a slower speed puts a small amount of power back into the battery and increases overall efficiency.