In this Blog Post, I have accessed the Google Gemini API from Salesforce Apex using the API Key approach.

Sample Apex Code:

public class GeminiAPIController {
    
    public static String invokePrompt( String strMessage ) {
        
        // Setting default value to return
        String strResponse = 'Some issue with the API. Please try again later.';
        // End Point of the API
        String strEndpoint = 'https://generativelanguage.googleapis.com/v1beta/models/gemini-2.0-flash:generateContent?key=<YOUR API KEY>';
        
        HTTP h = new HTTP();
        HTTPRequest req = new HTTPRequest();
        req.setEndPoint( 
            strEndpoint 
        );
        req.setMethod( 
            'POST' 
        );
        req.setHeader( 
            'Content-Type', 
            'application/json' 
        );       
        req.setBody( 
            '{ "contents": [ { "parts":[ {"text": "' + 
            strMessage + '" } ] } ] }'
        );
        HTTPResponse response = h.send(
            req
        );
        System.debug( 
            'Response is ' + 
            response.getBody() 
        );
        GeminiAPIWrapper objWrap = ( GeminiAPIWrapper )System.JSON.deserialize( 
            response.getBody(), 
            GeminiAPIWrapper.class 
        );
        
        if ( objWrap.candidates.size() > 0 ) {
            
            for ( GeminiAPICandidateWrapper objCaWrap : objWrap.candidates ) {
                
                GeminiAPIContentWrapper objCoWrap = objCaWrap.content;
                
                if ( objCoWrap != null ) {
                    
                    if ( objCoWrap.parts.size() > 0 ) {
                        
                        for ( GeminiAPIPartWrapper objPWrap : objCoWrap.parts ) {
                            
                            System.debug( 'Text: ' + objPWrap.text );
                            // Retrieving the response for the Prompt
                            strResponse = objPWrap.text;
                            
                        }
                        
                    }
                    
                }
                
            }
            
        }
        
        return strResponse;
        
    }
    
    // Wrapper Classes to handle the API Response
    public class GeminiAPIWrapper {
        List < GeminiAPICandidateWrapper > candidates;
    }
    
    public class GeminiAPICandidateWrapper {        
        GeminiAPIContentWrapper content;
    }
    
    public class GeminiAPIContentWrapper {        
        List < GeminiAPIPartWrapper > parts;
    }
    
    public class GeminiAPIPartWrapper {
        String text;
    }
    
}

Please add the Gemini API Endpoint URL in the Salesforce Remote Site Settings so that the HTTP Request to it will succeed. Make sure the Remote Site Setting is Active.