gdb actually has a "gui" mode, try
gdb --tui -q your_program_name
then the main.cpp is supposed to show up on top, you can use up/down arrow keys on ur keyboard to browse the file.
if you'd like to switch to the other cpp file, simply do:
(gdb)l func.cpp:1
adding a breakpoint is through
(gdb)b line_number or function_name