Tags are available in G2G, but are not available for use on profiles, except that by following a surname tag you can see editing activity for profiles in that surname. Users can follow tags, but can't follow templates or categories.
Templates have been created for certain special purposes. A template generally creates a formatted display on the profile where it is used and it often also automagically places the profile into one or more categories. Templates are not available for all situations where a person might want to use a category, and they aren't available in G2G. Only leaders can create or edit templates.
Categories can be used on profiles, but are not used in G2G. Any contributor can create or edit a category, but the Categorization project has guidelines for their use.