IntegrationResponse SAM Template - aws-lambda

Question

Questions:
IntegrationResponse to Sam template ? Is it possible without OpenApi?
Way to add headers to GatewayResponses in SAM template?
What im trying to achive:
Define gateway responses and integration responses inside a SAM
CloudFront template.
What i checked so far:
SAM github link
SAM issue
I was checking SAM github but for me it wasnt clear how to do it based on the above link.
Also i didnt found any answer on stackoverflow which would explain why the headers are bad in my gatewayresponse snippet
Every Help is appreciated
Some examples:
Globals:
Api:
GatewayResponses:
MISSING_AUTHENTICATION_TOKEN:
ResponseParameters:
# gatewayresponse.header.Cache-Control: "no-cache"
ResponseTemplates:
"application/json" : '{"errors": [{errorKey: "error Value"}]}'
StatusCode: '404'
#defaultResponse: true
Expected function level integrationResponse:
Function:
Type: AWS::Serverless::Function
Properties:
Handler: Function.handler
Timeout: 20
CodeUri: "src/Function"
Role: .......
Events:
FunctionGet:
Type: Api
Properties:
Path: /Function
Method: get
IntegrationResponse:
SOME_ERROR_CODE
ResponseTemplates
integrationresponse.header

Answer 1

So long story short half of my question is stupid. in proxy integration API GW is by defult returning the response from server to client no need further declaration in SAM template.
As for the headers the following way is the correct:
Globals:
Api:
GatewayResponses:
MISSING_AUTHENTICATION_TOKEN:
ResponseParameters:
Headers:
Access-Control-Allow-Origin: "'*'"
Access-Control-Allow-Headers: "'*'"
Cache-Control: "'no-cache'"
Content-Type: "'application/json'"
ResponseTemplates:
"application/json" : '{"errors": [{errorKey: "error Value"}]}'
StatusCode: '404'
#defaultResponse: true