Aliases are shell-specific – in this case, most likely bash-specific. To execute an alias, you need to execute bash, but aliases are only loaded for interactive shells (more precisely, .bashrc will only be read for an interactive shell).
bash -i runs an interactive shell (and sources .bashrc).
bash -c cmd runs cmd.
Put them together:
bash -ic cmd runs cmd in an interactive shell, where cmd can be a bash function/alias defined in your .bashrc.
find -name \*bar\* | xargs bash -ic gi foo
should do what you want.
Edit: I see you’ve tagged the question as “tcsh”, so the bash-specific solution is not applicable. With tcsh, you dont need the -i, as it appears to read .tcshrc unless you give -f.
Try this:
find -name \*bar\* | xargs tcsh -c gi foo
It worked for my basic testing.