Usually you set up environment variables with the shell and they are inherited by all the other programs you run.

When debugging, it can be useful to try running your program with a modified environment without having to start GDB over again.
 

path directory
Add directory to the front of the PATH environment variable (the search path for executables), for both GDB and your program. You may specify several directory names, separated by ‘:’ or a whitespace. If directory is already in the path, it is moved to the front, so it is searched sooner.

You can use the string ‘$cwd’ to refer to whatever is the current working directory at the time GDB searches the path. If you use ‘.’ instead, it refers to the directory where you executed the path command. GDB replaces ‘.’ in the directory argument (with the current path) before adding directory to the search path.

show paths

Display the list of search paths for executables (the PATH environment variable).

show environment [varname]

Print the value of environment variable varname to be given to your program when it starts. If you do not supply varname, print the names and values of all environment variables to be given to your program. You can abbreviate environment as env.

set environment varname[=] value

Set environment variable, varname, to value. The value changes for your program only, not for GDB itself. value may be any string; the values of environment variables are just strings, and any interpretation is supplied by your program itself. The value parameter is optional; if it is eliminated, the variable is set to a null value. For example, the command, set env USER = foo, tells a Unix program, when run, that its user is named ‘foo’. (The spaces around ‘=’ are used for clarity here; they are not actually required.)

unset environment varname

Remove variable, varname, from the environment to be passed to your program. This is different from ‘set env varname =’; unset environment removes the variable from the environment, rather than assigning it an empty value.

Warning:
GDB runs your program using the shell indicated by your SHELL environment variable if it exists (or /bin/sh if not). If your SHELL variable names a shell that runs an initialization file—such as ‘.cshrc’ for C-shell, or ‘.bashrc’ for BASH—any variables you set in that file affect your program. You may wish to move setting of environment variables to files that are only run when you sign on, such as ‘.login’ or ‘.profile’.