What sort of math questions appear most on the GMAT? any particular areas to study?

Number properties (all the stuff before algebra, geometry, etc, such as divisibility, primes, exponents...). Basic 1st-year algebra (linear eqns, quadratic eqns, inequalities, etc). Standard word problems (rates, work, ratios, etc). Basic statistics (mean, median).

Also expect to get geometry and fractions, decimals and percents problems - though perhaps not as many as the above.

Occasionally combination / permutation, probability, overlapping sets, standard deviation - not very common in general though.