How to Type Clitic Examples with the gb4e Package

To fix this, you have to use the ensuremath{} command. The following MWE actually works:

documentclass[12pt, letterpaper]{article}

usepackage{forest}

usepackage{gb4e}

begin{document}

begin{exe}
ex Enclisis 
label{EnclisisorLocativeCase1}
begin{center}
begin{forest}
[PP [DP [NP [N[pahá,tier=word]]] [D[-ø,tier=word]]] [P[ensuremath{=}ta,tier=word]]]
end{forest}
end{center}
end{exe}

end{document}

As David Carlisle says in the comment, there's no need for ensuremath here, and in fact you probably don't want it. You simply need to protect the = sign from the forest parsing mechanism. And the normal way to do that is to simply enclose the =ta (or even just the =) in {..}.

Also, you shouldn't use to get a new line in an example like this (as opposed to within a gll line, of course, in which is a delimiter.)

If you really need a blank line, or you want to put unnumbered material below an example, you should use the sn macro (which stands for 'sin número' = without number).

documentclass[12pt, letterpaper]{article}

usepackage{forest}

usepackage{gb4e}

begin{document}

begin{exe}
ex Enclisis
label{EnclisisorLocativeCase1}
snbegin{center}
begin{forest}
[PP [DP [NP [N[pahá,tier=word]]] [D[-ø,tier=word]]] [P[{=ta},tier=word]]]
end{forest}
end{center}
end{exe}

end{document}