I cannot come up with a one-word expression for 'making a slope gentler' and the opposite expression also (making a slope steeper). For example, it may be less redundant to express 'Aing a slope', if there is a verb A to indicate making it gentler. Could anyone recommend some expressions?
Levelling is a word used to make something more level.
To use it in some sentences close to your question:
The slope was too steep so I levelled it off a bit.
Adrian thought the slop was too steep so he's spending the afternoon levelling it off.
The opposite could be raising.
It doesn't work perfectly and when I try saying it out loud I'd go for a longer form but this word would achieve your goal.
There is the verb to grade:
To level or smooth to a desired or horizontal gradient:
Bulldozers graded the road.
Usually it has to do with making the ground slope at the correct amount of steepness, but it can be used for other things. For example:
Without background activity (dashed line), the probability was all-or-none, reflecting the threshold for action potentials. With background activity (continuous line), the response curve was graded and there was a non-null response for subthreshold inputs (<0.2 mS/cm2).
You can also ease a slope.
The slope of the driveway is too steep. It needs to be eased somewhat. We would like it to be more gradual.