Connecting Google Sheets to Insightly to allow Organisations and Contacts to be accessed in Sheets

I need code to connect 2 Google Sheet to Insightly so that an accurate an up to date list of: 1. Insightly Contacts, and; 2. Insightly Organisations, is created and maintained in Google Sheets. The integration needs to include importing Insightly custom fields. Where Organisation details or Contact details are modified in Insightly these changes need to be reflected in Google Sheets. It is not necessary for changes in Google Sheets to be reflected in Insightly. The code to achieve this integration must be developed in Google Sheets.