Hi,
if you are a phpbb forum admin you may experienced one issue with Gym Sitemaps Mod installation.
When you clicked the link News or Site map under some Forum folder, you got:
template->_tpl_load_file(): File ./styles/yourforumtheme/template/gym_sitemaps/display_topics_list.html does not exist or is empty
or something like that..
The reason is that Gym Sitemaps have the folder /themename/template/gym_sitemaps/ added in prosilver or subsilver theme directory which is the default phpbb themes.
You need to download the folder out of this directory and upload it into your actual theme. In my case it was SimpleCorp
You may need to Purge cache or refresh theme, but i dont think so..
Problem solved...