Using the markup %%Variable you can directly assign the result of an expression to an existing question. Please note that the questions type must be compatible to the result of the expression, otherwise an error is signaled and the assignment will not be created. Unlike using the expressions in rules, no eval(...) is required. The assignment is always kept up-to-date, so the value of the assigned question is never outdated.
%%Variable myGradient = gradient( measurement[-10s,0s] )