In order to have a function that simplifies sums
in a general context (hypergeometric) we need to use
both Zeilberger's algorithm (in "Zeilberger")
and Hyper's algorithm (in "solve_rec").
GosperSum (as well as nusum) only deal
with a very small set of "easy" examples.
Remark: nusum is not necessary any more
and could be substituted by "Zeilberger"
by for historical reason it could remain
in Maxima (Gosper himself wrote the implementation).
Zeilberger's and Hyper's algorithms can
be indeed very time consuming but there
is no way out if you want to solve
non-trivial sums.
Fabrizio