Difference in output when f77 code compiled with ifort & gfortran

Difference in output when f77 code compiled with ifort & gfortran

I need some pointers to solve a problem that I can describe only in a
limited way. I got a code written in f77 from a senior scientist. I can't
give the code on a public forum for ownership issues. It's not big (750
lines) but given implicit declarations and gotos statements, it is very
unreadable. Hence I am having trouble finding out the source of error.
Here is the problem:
When I compile the code with ifort, it runs fine and gives me sensible
numbers but when I compile it with gfortran, it compiles fine but does not
give me the right answer. The code is a numerical root finder for a
complex plasma physics problem. The ifort compiled version finds the root
but the gfortran compiled version fails to find the root.
Any ideas on how to proceed looking for a solution? I will update the
question to reflect the actual problem when I find one.