How to use Gemini API in laravel 5.4 project, without package installation

Gemini API
1

I have created this service for using Gemini API, and trying to send request, but get this
Client error: POST https://api.gemini.com/v1/completions resulted in a 404 Not Found response:
{“result”:“error”,“reason”:“EndpointNotFound”,“message”:“API entry point /v1/completions not found”}
class GeminiService
{
protected $client;

public function __construct() {
    $this->client = new Client([
        'base_uri' => 'https://api.gemini.com/v1/',
        'headers' => [
            'Authorization' => 'Bearer ' . config('gemini.api_key'),
            'Content-Type' => 'application/json',
        ],
    ]);
}

public function getTextResponse($prompt) {

    $response = $this->client->post('completions', [
        'json' => [
            'prompt' => $prompt,
            'model'=> "gemini-1.5-flash"
        ],
    ]);

    return json_decode($response->getBody(), true);
}

}
Can someone provide me the correct API url for this kind of calls?