At first, you may run the program and debug it by pid. This solution, of course, doesn't cover all cases.
Another approach is to use Linux capabilities for inter-process communication. In short, you redirect the output of ret to a FIFO special file ("named pipe") and then read from that FIFO via debugger. Here's how it's done. From bash, run:
mkfifo foo
This creates a special file in your directory that will serve as a named pipe. When you write text to this file (using the same syntax echo "Hello" >foo), the writing program will block until someone reads the data from the file (cat <foo, for instance). In our case, a gdb-controlled process will read from this file.
After you created a fifo, run from bash:
ret > foo & # ampersand because it may block as nobody is reading from foo
gdb blah
Then, in gdb prompt, run
run <foo
And get the desired effect. Note that you can't read the data from the fifo (as well as from a usual pipe) twice: when you've read all the data, the blah process dies and you should repeat the command writing to foo (you may do it from the other shell window).
When you're done, remove the fifo with rm foo (or place it into the directory where it will automatically be removed upon system restart, such as /tmp).
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