Use GPI when targeting large, enterprise-level companies, for strategic decision-making, and to leverage Gartner’s influence. Buyers using GPI are typically more in the research phase of their journey, so the more robust reviews tend to carry more weight there. However, because of their length, you'll generally have to put in more effort (or $) to get customers to do them.
Use G2 when aiming for higher review volume, more diverse demographics, or when targeting SMBs or global markets. Also, prioritize G2 if online visibility and user experience comparisons are key to your sales process (e.g. badges to put on your website, rankings, etc.)