\Gls
-like commands use \makefirstuc
to capitalize only the first letter of a sentence.
I've redefined the meaning of \makefirstuc
to be the same of \capitalisewords
which instead capitalizes all words in a sentence.
So, add this in the preamble
\makeatletter
\let\oldmakefirstuc\makefirstuc
\renewcommand*{\makefirstuc}[1]{%
\def\gls@add@space{}%
\mfu@capitalisewords#1 \@nil\mfu@endcap
}
\def\mfu@capitalisewords#1 #2\mfu@endcap{%
\def\mfu@cap@first{#1}%
\def\mfu@cap@second{#2}%
\gls@add@space
\oldmakefirstuc{#1}%
\def\gls@add@space{ }%
\ifx\mfu@cap@second\@nnil
\let\next@mfu@cap\mfu@noop
\else
\let\next@mfu@cap\mfu@capitalisewords
\fi
\next@mfu@cap#2\mfu@endcap
}
\makeatother
and everything should be as you want.
Complete MWE:
\documentclass[a4paper,12pt]{article}
\usepackage{glossaries}
\makeatletter
\let\oldmakefirstuc\makefirstuc
\renewcommand*{\makefirstuc}[1]{%
\def\gls@add@space{}%
\mfu@capitalisewords#1 \@nil\mfu@endcap
}
\def\mfu@capitalisewords#1 #2\mfu@endcap{%
\def\mfu@cap@first{#1}%
\def\mfu@cap@second{#2}%
\gls@add@space
\oldmakefirstuc{#1}%
\def\gls@add@space{ }%
\ifx\mfu@cap@second\@nnil
\let\next@mfu@cap\mfu@noop
\else
\let\next@mfu@cap\mfu@capitalisewords
\fi
\next@mfu@cap#2\mfu@endcap
}
\makeatother
\newacronym{tla}{TLA}{three lettered acronym}
\makeglossaries
\begin{document}
\glsfirst{tla}
\Glsfirst{tla}
\end{document}
Output:
