Personal gain is not appropriate when you seek only for your own good and not the good of all.  When you seek personal gain selfishly or for only you it is not for the good of all.

When you seek personal gain, so that you may live and help others and live to serve the God/Goddess within and to help All as One this is when personal gain is good.

Ask yourself, am I asking for this for only me, or am I asking for this for me in order that I may help others also.

When I think of the concept "personal gain", it seems to emit a sense of negativity, a wrongness almost. In the way the term is used in most modern literature and different person/s interpretations, I can see how you have easily become confused.
Personally I believe in Karma's concepts rather than 'personal gain'. By this I mean, the work you perform within the strand of time your life travels upon, within the weave of the verse, will reward or punish you as you have done yourself.
If you feel that your act improves or gives you some form of gain, ask the following: is this to reward your own greed? Look at the intent behind what you seek to do. If someone else was to do as you are about to, would you approve?

Hope this helps a little!
xxKylaxx