Using property list from expl3

The correct way is with \prop_item:Nn:

\documentclass{article}
\usepackage{expl3}
\usepackage{xparse}

\ExplSyntaxOn
% define a propery list
\prop_new:N \g_leal_tooltip_prop
%
\NewDocumentCommand{\CSet}{mm}
 {
  \prop_gput:Nnn \g_leal_tooltip_prop {#1} {#2}
 }
%
\NewDocumentCommand{\CGet}{m}
 {
  \prop_item:Nn \g_leal_tooltip_prop {#1}
 }
\ExplSyntaxOff

\begin{document}
\pagestyle{empty}

Hello

\CSet{key1}{value1}
\CGet{key1}

\CSet{key2}{value2}
\CGet{key2}

\end{document}

The \prop_item:Nn function has been introduced on 2014-07-17; if you get it is undefined, your TeX distribution is lagging behind.

An almost equivalent version would be with the help of a token list variable:

\tl_new:N \l_leal_tooltip_temp_tl
\NewDocumentCommand{\CGet}{m}
 {
  \prop_get:NnN \g_leal_tooltip_prop {#1} \l_leal_tooltip_temp_tl
  \tl_use:N \l_leal_tooltip_temp_tl
 }