Be careful with what kind of gel pen you use. My husband dropped a gel pen on an expensive dress shirt he'd left laying on the floor, and the shirt wicked up enough ink to make a 2" stain. I tried everything I could think of on that stain and even did an hour's worth of research on the internet. I found that law enforcement recommends signing checks with gel ink pens because it's the one kind of ink that thieves cannot bleach out of checks.
Having said that, I agree with the others who say if you're going to cut on that line, whatever you use won't be a problem because the line will be inside the seam allowance.