Scratch

Here I fixed it with a simple piece of coding - http://scratch.mit.edu/projects/Ztype_Test/2793951 - just download - I have put a comment on where the new coding is so you don't have to put up with the "based on Ztype's Project".

There are quite a few improvements that could be made. Let me give you a few gravity scripts.

Here is the gravity.

when gf clicked
set [gravity v] to (-.4)
set [velocity v] to (0)
forever 
if <touching [ground v]?>
set [velocity v] to (0)
else
change [velocity v] by (gravity)
change y by (velocity)

Here is jumping to work with gravity

when gf clicked
forever if <key [space v] pressed?>
if <(yvelocity) = (0)>
set [jumping v] to (1)
repeat (10)
change [yvelocity v] by <(gravity) * (-1.4)>
end
set [jumping v] to (0)

This is just one of the things you can do. It will prevent your character from flying, it will add velocity to your jumping and falling, and you can easily edit it.