80W90 is a multiviscosity version of the single-viscosity 90 gear oil; it's the same concept as 10W30 motor oil being the multiviscosity version of single-viscosity 30-weight motor oil. The "W" is a "winter" rating, meaning that the oil in question is designed to be usable in winter months in cold climates.
80W90 oil is perfectly satisfactory for your purposes in a cement mixer - it can be either GL4 (with very little molybdenum disulfide) or GL5 (with lots of moly-d)... for a cement mixer I'd recommend using the GL5 because you have no brass synchro components for the moly-d to attack. GL4 is for older manual transmissions that can't tolerate moly-d because they contain brass. For gearboxes that can use it, moly-d adds extra protection against wear, especially in cases where regular maintenance is unlikely.