Tax liability query in gst

there is one  firm has gstn in mumbai - with no income in gst

it also has branch in surat - with rent income only, but no gst registration in surat and the rent income is not exceeding Rs. 20 lakhs

in income tax pan india base only rent income, 

then what will be the  liability in rent income in surat having no gstn ?

whether surat rent income required to be tax with mumbai gstn?