\documentclass{article}
\usepackage{glossaries}

\makeglossary
\begin{document}

% gf on 'inp.tex' works fine
\input{inp.tex}
% \input{../texinclude/inp.tex}

% completion after type '\ref{' works if aux-file exists
\ref{sec:bar}

% completion after type \gls{' didn't work
\gls{g:foo}

% but completion works if used '\input{../texinclude/inp.tex}

\printglossaries

\end{document}
