You can probably get better information from one of the Gravely websites like the Yahoo Gravely Club
Your pictures show the tractor to be a 1959 model "LI". The "I" stands for intermediate speed, and is considered more desirable than the "L" (which moves too fast for use with many attachments.)
Your pictures don't show if the tractor has the optional governor which runs off the fan belt. The governor is a very desirable option.
The pictures don't clearly show how the blade is mounted. It DOESN'T look like the normal mounting for a 1959 model. It looks like you may have a quick hitch adapter bolted to the tractor, and probably a quick hitch snow blade. The quick hitch adapter and mounting for the quick hitch blade may be more valuable than the tractor itself.
Value seems to depend on location. Gravely's seem to be more common in the Eastern part of the county. They are less common from Indiana west.
Where are you located?