This is a variation of the modal "Interpolate" operator, where a series of keyframes is generated according in stead of a single keyframe. The behavior should be the same as the GPv2 operator. Much of the code is shared between the "Interpolate" and "Interpolate Sequence" operators now (utility functions section at the top). Pull Request: https://projects.blender.org/blender/blender/pulls/127709