GConvert uses external plug-ins to implement more formats for saving icons. You can manage plug-ins using the Plug-In Manager in the Environment Options.
Plug-ins are special DLL library files that were programmed to interact with GConvert.
The original distribution is shipped only with two external plug-ins to support JPEG and GIF files. Other plug-ins may be available freely for download from www.gdgsoft.com.
Normally plug-ins available for download are shipped in Self-Installing packages so they can be automatically set up in order to be handled by GConvert. But this tab allows you to add or remove existing plug-ins manually.
Please check the GConvert's Web page for other plug-ins, freely available to download.