I'm a similar weight, although a handful of years older, and I guess you just want to gain muscle rather than weight per se.
I strongly recommend taking on some additional protein to ensure that your diet is giving your body all the protein needed for muscle growth. It doesn't have to be whey protein, but the commercially available recovery drinks will contain this, are convenient to use and contain other vitamins and amino acids etc. to aid recovery. I generally have one within 15 minutes of coming in and then some other protein (aiming for at least 20g) an hour later. My weight has stayed quite stable doing this and fat has decreased (although I'm not aiming to put on weight so don't always eat much on the ride itself).
I don't know enough about Creatine to really comment, but I believe it helps muscle time to exhaustion and is therefore most suited to strength training. There are some concerns about long term side affects, I believe, so if you're just doing endurance rides I wouldn't bother.
If you just want to put on weight, regardless of whether it's lean muscle or fat, skip breakfast every day (which will slow your metabolism down) and eat ice cream with every meal.