What is the site URL?
https://labelx--uat.sandbox.my.salesforce-sites.com/GF/services/apexrest/IGJ
...
{"Data":[{"GLLedger":"GLLedger","GLAccount":"GLAccount","DocumentNo":"DocumentNo","CurrencyCode":"CurrencyCode","AccountingVariable4":"AccountingVariable4","AccountingVariable3":"AccountingVariable3","AccountingVariable2":"AccountingVariable2","AccountingVariable1":"AccountingVariable1","Account":"Account","Description":"Description","GLAmount":1,"Amount":2}],"UserName":"szhuo@goldfinchcloudsolutions.com"}
Which object does it link to?
GoldFinch Integration Staging
JSON Size Limitations
The absolute limit is the Heap Limit (6MB for synchronous code, 12MB for asynchronous code: Execution Governors and Limits), but it takes special planning to deal with parsing the JSON without blowing up the heap limits. Most developers barely manage to parse something maybe half the size, but it's possible to do so if you immediately destroy the string when you're done with it.