You can do this by virtual populate, introduced in mongoose version 4.5 . For that you need to define a virtual field in mongoose schema.
var GallerySchema = new mongoose.Schema({
    name: String,
    objectId: {
        type: mongoose.Schema.Types.ObjectId
    },
});
GallerySchema.virtual('user', {
    ref: 'User',
    localField: 'objectId', 
    foreignField: '_id' 
});
Ans when you run find  query, just populate it with user.
Gallry.find({_id: galleryId}).populate('user','firstName lastName').exec(function(error, gallery) {
    console.log(error);
    console.log(gallery);;
});
Above code is not tested in program, there may be typos, You can get more details about mongoose virtual populate on below link
http://mongoosejs.com/docs/populate.html