Vat credit on exports in gst

We are registered as manufacturer in Gujarat with Commercial Tax Department. We were involved in Domestic sales & Exports since last four years. However domestic sales has been on substantially lower side over a period of last two years. Due to this unutilised VAT credit of inputs has been carried forward in our VAT Return since last two years for a huge amount. Now with the introduction of GST what should be done with this credit. Will this credit be allowed to be carried forward from VAT to GST? Is it possible to claim refund of this VAT credit? If refund can be claimed what is the time limit for claiming any such refund? Further can be refund be claimed even if assessment for VAT has been completed for a particular year?