What are the key considerations when customizing a Genesis Croix de Fer 50 for gravel riding, and how can you balance the need for a comfortable, endurance-focused bike with the desire for a more aggressive, responsive ride? Specifically, what modifications to the frame, wheels, and gearing would be most effective in creating a versatile, high-performance gravel bike that can handle everything from smooth dirt roads to technical singletrack? Would a 1x or 2x drivetrain be more suitable for this type of riding, and what are the trade-offs between a wider, more stable tire and a narrower, faster-rolling option?