Can studying a Guru's biography be done for personal gain, and how does that compare to doing it for spiritual reasons?

Yes, Chapter 0 clarifies that the study of a Guru's biography can be approached with two different motivations: 'Kamya' (with desire) and 'Nishkam' (with a selfless mind). A 'Kamya' approach is when the study is undertaken for worldly prosperity or for the removal of worries, calamities, distress, and diseases. In contrast, a 'Nishkam' approach is undertaken for the ultimate goal of liberation from the world and freedom from the cycle of birth and death. The text makes it very clear that Nishkam Guru service, which is selfless, will always be considered superior, regardless of the specific form the service takes.