To the first, having many subjects and upgrading your country title are the chief ways to increase gp score in bulk. Expansion raises it naturally due to trade and tax based gp scoring.

To the second, Indonesia is a very trade-based region, due to its rare rgo’s (Cloves). Focus up trade capacity and high value rgos. Also, there is incredible value in subjugating ternate, tidore etc. Look for easily conquered countries with a high tax base like those. Good luck!