Also I have two types of items. First ones have 18% gst and second ones have 28% gst. How can add this on the category level so that invoice shows correct tax information?

StoreHippo provides extensive support for GST on the platform. We cover various aspects of GST with our flexible tax engine to help you achieve comprehensive GST compliance. You can add different GST slabs to taxes.

Please refer the link to add GST tax rules to your store:
http://help.storehippo.com/topic/setting-up-gst-of-india