debugging f77 under linux with/without GUI

Hi everybody,
Have anyone tried a good debugger, with GUI in preference, for f77 code
with hundreds of subroutines ? under linux !
I've downloaded lots of GUIs like: DDD, Kdebug ... all of them are
interfaces for GDB. I don't know any other free debugger. I want to
have an idea about them before investing my time in on of them.
thanx