IGST on Commercial Rental Income (Different State)

• Can they use Karnataka GSTIN? No, because the place of supply is Gujarat where the immovable property is located.

• Is Gujarat registration required? Yes, standard GST provisions require registration in the State where the taxable service originates (location of the property), making cross-state billing from Karnataka legally incorrect.

No , seprate GST registration required in Gujarat , if you do not have any fixed establishment in Gujarat , you can charge IGST in the tax invoice for providing rental services 

Relavent sections of the IGST Act 2017 

1) section 12(3) IGST Act 2017  , place of supply location of the property located that is  Gujarat . 

2) section 7(3) IGST Act 2017 ,  when supplier & receipent  locations  are   in different state IGST applicable . 

3) section 5(1) provide levy of IGST on inter-State supply 

4) section 31 CGST Act 2017  , issue of tax invoice by the registered supplier , Can issue to two  different tax invoice  as per  respective shares in the rent  . 

Please refer all these section .Take more opinion as well 

You also can take advance ruling under section 97(2)(f) of the CGST 2017 , for registration in Gujarat and issue of tax invoice .