Automated decision-making techniques are useful to support engineers when performing requirements engineering tasks. However, to be effectively used in practice they need to be integrated into the organisational context, in which stakeholder engagement becomes a critical adoption factor. In this paper, we propose a tool-supported collaborative requirements prioritisation process, called GRP, which exploits gamification elements to engage distributed stakeholders to contribute to the overall decision-making process. Analytic Hierarchy Process is used as key component of the game engine, and enables an iterative prioritisation process. The GRP process has been evaluated through an exploratory case study, which has been conducted at a small software company, providing us with preliminary evidence about the effectiveness of the proposed solution. The main findings and lessons learned from the case study are presented.