Leave your grade point average out of your cover letter and resume. In many cases, hiring managers won't know your GPA unless you tell them. This is one way to focus the attention on your other qualities, such as leadership experience, campus involvement, or past internships.
Use a calculator to figure out your GPA for a short time frame, leaving out a troublesome semester if necessary. This is a great tactic if you absolutely must indicate your GPA to be considered for the banking internship. You can submit your GPA for the previous year, for example, if you're knee deep in a particularly challenging semester. Or you can use an overall GPA if it's higher than one that's impacted for certain courses.
Focus your resume and cover letter on any of your attributes or circumstances that affect your GPA. For instance, if you're an athlete, mention your dedication to your team and training schedule. If you work long hours to support yourself, mention your self-sufficiency and independence. This is one way to turn a negative into a positive and market yourself as more than just a grade point average.
Rehearse an honest, yet brief explanation for your low GPA to use during your interview should the hiring manager ask. It's important that you don't sound like you're making excuses for yourself. Instead of saying "My professors were tough and I didn't have time to study," say "I took on more than I could handle with my role in social clubs and organizations last semester."
Sell yourself by following up with a statement on what your low GPA has taught you. This will impress the hiring manager at a banking institution since it will demonstrate that you're willing to learn from mistakes and that you take your education seriously. You can say "I've taken time to refocus on my education and I'm excited to intern in the field that I wish to work in." Or, "My time management skills have improved tremendously and I look forward to finishing out this semester with a much better grade point average." Keep in mind that the ideal candidate isn't one with a high GPA. It's one who has the capacity to learn and commit to excellence.