The fact that one is ISO 32 and the other is ISO 46 means that they are not the same viscosity! One has a viscosity of 32 centistokes at 40 degrees C and the other - well work that out for yourselves. INTERNATIONAL (get with it!) Standards Organisation (ISO) viscosities are quoted at 40C.
When you say "gearbox", I take it you don't mean a geared headstock, just the open gears, either change wheel or lever change for screwcutting / feeds? If that is the case, the only difference it could make is the feed rate via wick feeds, but don't fret, a few degrees temperature difference will make the viscosities the same. The more viscous oil will stay in place longer, so doesn't need such a high feed rate, anyway. For what its worth, I use ISO 220 oil normally used for the bearings of steam locos on everything except the wick feeds in the apron (ISO 32 hydraulic oil) and headstock bearings (ISO 22 spindle oil) except in sub zero (C) in our unheated workshop, when I pick up the 32 oil can instead of the 220.
If you are talking about an enclosed geared headstock, the small change could give rise to 2 problems, namely overheating, the big hazard being a thermal runaway of a bearing from a VERY cold start straight to continuous high speed, and more slip at wet clutches and brakes. It seems hardly enough of an increase to be any worry for hobby use, so yes, in your position I would buy the 46 vis if the other was only available locally in ridiculously large volumes at great cost. If you do much work though, you'll find that a gallon isn't such an imposition, especially if you're talking about a US gallon rather than the larger Imperial gallon (not used any more - "gallon" containers for everything liquid in the UK are now either 4 or 5 litres). These oils should be CHEAP, anyway, as they are unsophisticated. About the only additive worth mentioning might be anti rust, since they are operating at very moderate pressures and little above room temperature.
Oil data sheets quote viscosities at 40 and 100C. With those 2 you can determine the actual viscosity of an oil at any temperature between solidification and vaporisation using the attached spreadsheet. Put the published oil data in the green cells and your answer appears in the blue cell. Don't mess with the rest, except maybe the pale green cells if you have non standard temperatures for input data. Works for multi and monogrades, anything.
Eddie
