Providing corporate guarantee per-se will not be providing of any service. However when against giving guarantee consideration is received, such amount will be for supply and liable to GST subject to other provisions. If no such consideration is received there is no taxable ‘supply’.
However, as per query it appears that the entities are related parties as per meaning given for said term in section 15 of CGST Act. Therefore, as per para 2 of Schedule 1 to CGST Act providing corporate guarantee without any consideration will still amount to ‘supply’.
The valuation will be required to be done as per Rule 28 of CGST Rules,2017 and the liability will be required to be discharged accordingly.