dökümanda guzzle üzerinden de proxy ile kullanabileceğiniz yazıyor:

use GeminiAPI\Client as GeminiClient;
use GeminiAPI\Resources\Parts\TextPart;
use GuzzleHttp\Client as GuzzleClient;

$guzzle = new GuzzleClient([
  'proxy' => 'http://localhost:8125',
]);

$client = new GeminiClient('GEMINI_API_KEY', $guzzle);
$response = $client->geminiPro()->generateContent(
    new TextPart('PHP in less than 100 chars')
);
curl için:

use GeminiAPI\Client;
use GeminiAPI\Resources\Parts\TextPart;
use GeminiAPI\Responses\GenerateContentResponse;

$callback = function (GenerateContentResponse $response): void {
    print $response->text();
};

$ch = curl_init();
curl_setopt($ch, \CURLOPT_PROXY, 'http://localhost:8125');

$client = new Client('GEMINI_API_KEY');
$client->withRequestHeaders([
        'User-Agent' => 'My Gemini-backed app'
    ])
    ->geminiPro()
    ->generateContentStream(
        $callback,
        [new TextPart('PHP in less than 100 chars')],
        $ch,
    );