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While you are chasing down Goverment/Manufactures information, a starting point for a guesstimate would be landing and takeoff charts for other airplanes. Looking at the charts for a 152 and a 172 (very different aircraft than yours) the takeoff roll is increased by 15% and the landing roll by 45% on dry grass as compared to dry pavement. These figures represent mainly the influence of the coefficient of friction of the landing surface. Thus, as an engineer (not aeronautical engineer) my first guess would be that your plane would behave similiarly. However, I want to repeat that this is just a starting guess and you should pursue people that would be more knowledgeable than me on the subject.
I would also think, from experience, that wet grass will greatly increase the rollout distance over dry grass. Also, from experience, wet grass doesn't affect takeoff distance over dry grass nearly as much as the height and density of the grass. That applies to dry grass verses pavement too.
Gravel, I think, could vary greatly depending upon the makeup and consolidation of the gravel, but would always be greater than dry pavement (another guesstimate as I do not have much experience on gravel). How it compares to grass again would depend on the makeup of the gravel.
The above statements were the Engineer talking. Below is the pilot talking.
My experience on grass is that each strip is different and that you can basically read the POH values and then throw away the book. Grass strips vary so much in surface roughness that the POH values are meaningless. My local airport has a very well kept grass strip. In a 152 or a 172 I can make landings on the grass that are much shorter than those predicted by the POH. However, if I go to a friends private strip the landing roll is considerably longer because the surface is rough and thus, hard to keep the wheels on the ground. Getting out is another story.
One suggestion. If you don't have alot of experience in unimproved field landings, I would find someone that does and learn from them. The big problem in many unimproved fields is the approach and departure path. The standard lefthand pattern is usually a rare exception. In the final analysis, landing and takeoff distance is all about pilot technique. With some good help and lots of practice you will be able to judge strips by flying over and looking at them and will know whether you and your plane are capable of getting in and out of a strip.
Good Luck.
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