The "Snippets" plugin will do exactly that. Depending on your platform and version of Gedit, it should already be included, in which case you can simply enable it by going to Edit -> Preferences -> Plugins tab.
If it is not present, you may need to upgrade Gedit, as it is a default plugin distributed with Gedit and I don't know of any way to obtain it separately. See http://projects.gnome.org/gedit/plugins.html.
To manage snippets, go to Tools -> Manage Snippets. One of the snippets already created for you is "Wrap selection in open/close tag." The snippet markup itself is:
<${1:p}>$GEDIT_SELECTED_TEXT</${1}>
and the trigger is Shift+Alt+W. You can easily copy this snippet to a new one and replace the tag and trigger to customize to your needs:
<${1:em}>$GEDIT_SELECTED_TEXT</${1}>
will wrap the selected text in "em" tags. You can even trigger snippets for tab completion, such as this one included for Java:
public static void main(String[] args) {
${1:System.exit(0)};
}
Simply typing "main" and pressing the Tab key will create the function body.