A gender gap usually refers to the differences in educational attainment by men and women, though it can have different meanings in different contexts. Additionally, in politics, this may refer to the real or perceived view that men and women have differing views on certain political issues.
The differences between men and women, while not immense, are easily seen in education. Currently, many coeducational colleges and graduate programs have student populations that are more than 50% female.
An apparent gender gap is one reason cited for Title IX funding in schools, to provide greater access for female students to sports, sporting venues, and competition. Title IX bans institutions with any amount of federal funding from discrimination on gender grounds.
In recent years, women have gained considerably in college. For example, in 1970, merely 42% of college students were women; in 2010, about 60% were women. While this may show a bias towards women in the admissions process, the change seems reflects more the later median marriage age  and women's national shifting of priorities from raising families to obtaining better jobs and careers.