My male coworker is paid nearly 20% more than I am and we do the same exact job! Is this gender discrimination?

Just like employers are prohibited from age discrimination and race discrimination, they are also prohibited from engaging in women discrimination in the workplace.  Specifically, employers are prohibited from discriminating against employees on the basis of race, color, religion, gender and national origin pursuant to Title VII of the Civil Rights Act of 1964. 

While you and your co-worker may perform substantially the same job, it is important to asses whether there are distinguishing factors other than gender that may account for the salary difference.  Does your co-worker have a master’s degree?  Does your co-worker have relevant experience that you did not have prior to joining the firm?  If there is truly nothing that differentiates you except gender, it may be worth taking a closer look at why your co-worker makes so much more money for performing substantially the same job.  There is no way to know for sure whether the difference is due to gender alone, but absent some other justification, you may be able to make the case that the disparity is due to gender discrimination.