Loading Google Compute/Kubernetes Engine hosted page in IFRAME

HubSpot CRM has iFrame Action type which loads page given in action URI (link for more info). Here's an example:

{
  "type": "IFRAME",
  "width": 890,
  "height": 748,
  "uri": "https://example.com/iframe-contents",
  "label": "Edit",
  "associatedObjectProperties": [
    "some_crm_property"
  ]
}

Basically, the content of the https://example.com/iframe-contents should be shown in iFrame.

When we host an app in Compute Engine or Kubernetes Engine (suppose default Hello World app) we will be given an External IP something like http://35.193.000.00/. When I replace this IP in the above iFrame uri the page is not loading in HubSpot nor any error is shown. What could be the potential problem? Is it GCP issue or HubSpot issue?
Is it possible that GCP does not allow the page to be loaded in iFrame?

Can someone please help me navigate to the right direction to debug this issue.