We propose principles for interpreting magnetic-bubble dynamics in a rotating-gradient experiment when no external in-plane field is present. If the orbit of the bubble is circular, simultaneous determinations of radius and phase lag with respect to the drive may be interpreted to measure the mobility coefficient and the velocity-momentum [V=V (P)] relation. Investigation of the stability of the circular orbit shows that in one limiting case the segments of V (P) having a positive slope will be accessible to experiment. We illustrate dependences of V on P expected for static bubbles with 1) small numbers of vertical Bloch lines, 2) horizontal Bloch lines, and 3) large numbers of vertical Bloch lines. We conclude that rotating-gradient measurements can determine the Bloch-line structure of the circulating bubble. The basically new element present in such experiments is the transverse component of effective bubble mass, which differs from the longitudinal component and will be much greater than the Döring mass whenever Bloch lines are present.