Yes, you can connect custom API providers using our HTTP API feature. For Gemini models, you can connect your own API keys by selecting the desired model from the dropdown, then clicking the Account dropdown and selecting "+ Add account."
Our HTTP API integration allows you to connect to any tool you have API access to, which would include Vertex AI for Gemini 2.5 Flash.
If you need any more help with connecting your custom API provider or using the Gemini 2.5 Flash model, please let me know. Would you like to provide more details about what you're trying to achieve or any specific questions you have?