import bpy


def gap_strips(strips, gap):
    strips = sorted(strips, key=lambda strip: strip.frame_start)

    for i in range(1, len(strips)):
        st0 = strips[i-1]
        st1 = strips[i]
        st1.frame_start = st0.frame_start + st0.frame_final_duration + gap

def gap_selected_strips(scn, gap):
    gap_strips( [ strip for strip in scn.sequence_editor.sequences if strip.select], gap)


gap_selected_strips(bpy.context.scene, 15)