Does Gold vs Platinum matter much when choosing a PSU?

Gold and Platinum describe efficiency, not overall build quality. A Platinum unit generally wastes a little less power converting electricity from the wall into power for the PC. The gap is only a few percentage points at the tested loads, and it varies with load and input voltage.

That rating alone doesn't tell you whether a PSU has good protections, handles power spikes well, or is a better purchase. I'd compare independent reviews of the exact models rather than assuming Platinum wins.

For most gaming builds, I wouldn't pay a big premium just for Platinum. The savings depend on how much power the PC draws, how long it's on, and your electricity price. A machine under heavy load around the clock has a better chance of recovering the extra cost than one used for occasional gaming.

If the prices are close, sure, take the efficiency upgrade if the rest of the unit checks out. Otherwise, Gold is fine.

Wattage and efficiency are separate questions. A good 750W unit can be enough for many gaming builds, but check the GPU manufacturer's PSU recommendation and the rest of your components before deciding. An 850W unit gives you more headroom, not automatically a better PSU.

Also check that the unit has the right connectors and enough cables for the GPU and anything else you're installing. I'd take a well reviewed Gold model with suitable capacity over a Platinum model that doesn't fit the build.

Look at the warranty too, especially when comparing similarly priced models. A longer warranty can be a useful sign of manufacturer support, but it isn't proof that one unit is better. A decent PSU can still have a shorter warranty, so check reviews and the terms rather than using warranty length as a hard cutoff.