Gay–straight alliance (or gender‐sexuality alliance; GSA) is a high‐school based club aimed at providing a safer environment for sexual and gender minority youth as well as their straight allies. Yet, as a club historically rooted in addressing sexual orientation‐related concerns, less attention has been given to understanding the changing relational dynamics of internal GSA activities aimed at expanding membership boundaries through the promotion of transgender inclusivity. I address this by bridging existing scholarship on GSAs, social movements, and the sociology of culture to showcase the impact boundary‐spanning strategies are having on GSA mobilization, in‐group solidarity, and external political and social activism. My findings reveal that membership boundary negotiations around gender diversity issues are shifting the social landscape of these clubs. Emerging barriers that impede boundary‐spanning efforts are also highlighted and discussed. More broadly, I generate new theoretical insights into how boundary spanning can shape political and social activism as well as offer promising future research directions in this area.