Final answer:
The hashtag used for the women's march in response to concerns about the Trump administration and gender equality was #WomensMarch.
Step-by-step explanation:
The hashtag used for the women's march organized in response to concerns about the Trump administration's stance on gender equality was #WomensMarch. The Women's March became a significant event in the history of gender equality movements, drawing attention to women's rights and social justice issues in the wake of the 2016 presidential election. The use of social media, particularly the hashtag, played a critical role in mobilizing and unifying participants from various locations, similar to other movements such as #MeToo and Black Lives Matter.