don't use toBuffer.

lambda-resize-gm.js

gm(imageBuffer.data).size(function(err,size){
  var w = 1200;
  var h = 630;
  
  if(err){
    console.log("gm error")
    context.fail(err);
  }

  this.background("#ffffff").resize(w,h).gravity("Center").extent(w,h).stream(function(err,stdout,stderr){
    if(err){
      console.log("gm process error");
      console.log(err,stdout,stderr)
      context.fail(err);
    }

    var chunks = [];

    stdout.on('data',function(chunk){
      chunks.push(chunk);
    });

    stdout.on('end',function(){
      var buffer = Buffer.concat(chunks);

      var params = {
        Bucket: BUCKET_NAME,
        Key: "hoge/hoge.png",
        ACL: 'public-read',
        ContentType: imageBuffer.type,
        Body: buffer
      };

      s3.putObject(params, function(err, data) {
        if(err){
          console.log("gm upload error");
          context.fail(err);
        }

        context.succeed({
          "error":false
        });

      });
    });

    stderr.on('data',function(data){
      console.log(`stderr ${size} data:`, data);
    })

  });
});