I'd bend on offset and include enough for the angle of rotation or the differnece to make it up the angle, but this not the point of this thread.
I'm sorry the use of this term is desciptive IMO, Gain reflects more to Radio usage, not angles and bends on first glance in the search engines...
Gain is also used in the "bending game". "Gain" is the proper vernacular terminology - Greenlee used to put out a booklet(assume they still do) called Conduit Bending Handbook. Mine is a 1990 edition, but let me tell you, it's solid gold for information right down to how much gain is increased for every degree!
Greenlee defines "Gain" this way - Because pipe bends in a radius and not at right angles, the length of pipe needed for a bend will not equalthe total length. Gain is the difference between the right angle distances A & B and the shorter distance C -- The length of pipe actually needed for th bend.
Gain Formula for 90 degree bends = 2 x radius -developed length or radius radius of bend x .43
See page 29 for any degree of of bend.
Just like ol' George Hart says - "Both time and energy will be saved if conduit can be cut, reamed and threaded before bending.