Here are a few tips on gdb.
Run foo with python-generated output:
$ ./foo `python -c 'print("hello")'`
$ ./foo $(python -c 'print("hello")') # same thing
$ python -c 'print("hello")' | ./foo # is this different? Hmmm.
You can also do this in gdb:
$ gdb ./foo
...
(gdb) set args $(python -c 'print("A"*32)')
Do you want to generate a really long line? Just use python:
$ python -c 'print("A"*32)'
Perhaps you want to write a really long line followed by some hex thingy, like an address?
$ python -c 'print("A"*1024 + "\xff\xee\xcc\xbb")
There are numerous gdb tutorials online. Here is a quick read with some essentials:
http://www.cs.toronto.edu/~krueger/csc209h/tut/gdb_tutorial.html