GitLab Integration Problem - Gitlab Behind Cloudflare ZeroTrust

### Environment

SaaS ([https://sentry.io/]())

### Steps to Reproduce

Hi,

To reproduce, Click "Add Installation"
Application added in Gitlab Self-managed behind Cloudflare Access/ZeroTrust. Given scope like api as asked and got application id/secret from gitlab.

In Sentry, added gitlab url etc, and when i click next, i initially got some json Parse error.

I temporarily then whitelisted Sentry IP address available but still getting this error:

Whoops! Looks like something went wrong! Give that another try.

An error occurred while validating your request.

### Expected Result

Installation should happen

### Actual Result

Whoops! Looks like something went wrong! Give that another try.

An error occurred while validating your request.

### Product Area

Other

### Link

[https://zingoy-rewards-pvt-ltd.sentry.io/settings/integrations/gitlab/?referrer=request_email]()

### DSN

*No response*

### Version

*No response*

The issue names no repository files, tests, or code entry points. Start by reproducing the GitLab self-managed installation behind Cloudflare Access and inspect the validation response and JSON parsing failure; done means the Sentry GitLab installation completes successfully in that setup.

Written by the indexing model from the issue text.