From the gb4e
- documentation:
All example-list commands have an optional argument which allows you
to set the labelwidth to, e.g., the widest label to come (...). So,
\begin{exe}[(234)]
will set the labelwidth to the width of
“(234)”—which is also the default. (...) The default labelwidth for
the exe- environment can also be changed globally by issuing a
\exewidth{<string>}
command in the preamble of your document
So, for your example, both of the following work (I also suggest to use small-caps as already demonstrated by Adam in his answer, as well as separate the language name from the translation with \hfill
unless explicitly required otherwise):
\documentclass{article}
\usepackage{gb4e,cgloss4e}
\begin{document}
\begin{exe}[(10.159)]
\exi{(10.159)} \gll b\'u \`a k\H{o} k\`am t\`aha k\'{\i} \\
\textsc{cl}2 \textsc{ds} (\textsc{b})hold (\textsc{a})break (\textsc{a})scatter \textsc{cl}7 \\
\glt ``They broke it off.'' \hfill (Missong)
\z
\end{document}
or
\documentclass{article}
\usepackage{gb4e,cgloss4e}
\exewidth{(10.159)}
\begin{document}
\begin{exe}
\exi{(10.159)} \gll b\'u \`a k\H{o} k\`am t\`aha k\'{\i} \\
\textsc{cl}2 \textsc{ds} (\textsc{b})hold (\textsc{a})break (\textsc{a})scatter \textsc{cl}7 \\
\glt ``They broke it off.'' \hfill (Missong)\\
\z
\end{document}