I am looking for a program that I can use to keep track of and calculate my grade in all my classes without going through and making a spreadsheet for each class.
If your school doesn't use any gradebook system you should consider it :)
Google found several linux projects like GradeL, GNota, etc. Try them. Imho "that works on gentoo" is not relevant while you can build almost everything from sources on GNU/linux and especially on Gentoo.
Probably you should consider some platform-independent web application too (google it). You can access your data from another computers (public, friend's), that's a big benefit.
I know you don't want a spreadsheets, but if I were you, I'll create one google docs spreadsheet with couple of sheet tabs.