The glossaries bundle is supplied with the package mfirstuc which provides the command:

This makes the first object of ⟨stuff ⟩ uppercase unless ⟨stuff ⟩ starts with a control sequence followed by a non-empty group, in which case the first object in the group is converted to uppercase. Examples:

• \makefirstuc{abc} produces Abc.
• \makefirstuc{\emph{abc}} produces Abc (\MakeUppercase has been applied to the letter “a” rather than \emph). Note however that
  \makefirstuc{{\em abc}}

  produces ABC (first object is {\em abc} so equivalent to \MakeUppercase{\em abc}), and

  {\makefirstuc{\em abc}}

  produces abc (\em doesn’t have an argument therefore first object is \em so equivalent to {\MakeUppercase{\em}abc}).

• \makefirstuc{{\’a}bc} produces Ábc.
• \makefirstuc{\ae bc} produces Æbc.
• \makefirstuc{{\ae}bc} produces Æbc.
• \makefirstuc{{ä}bc} produces Äbc.

Note that non-Latin or accented characters appearing at the start of the text must be placed in a group (even if you are using the inputenc package) due to expansion issues.

Note also that

produces: ABC. This is because the first object in the argument of \makefirstuc is \abc, so it does \MakeUppercase{\abc}. Whereas:

produces: Abc. There is a short cut command which will do this:

This is equivalent to \expandafter\makefirstuc\expandafter{⟨stuff ⟩}. So

produces: Abc.

If you want to use an alternative command to convert to uppercase, for example \MakeTextUppercase,¹ you can redefine \glsmakefirstuc. For example: