It's a good point that gold should start doing better than TIPS once negative real yields become entrenched. Of course in 2020 its done significantly worse. TIPS are supported by fed purchases and gold is confronted with active short selling market participants. I find often gold leads TIPS on the way down - it's more of a derivative asset: if people have a more neutral view on it it will fall quickly as players known its reflexive properties (ie people will form their fair price estimates of gold based on where it is going) will club it down.

The dollar exchange rate is the other input to the gold price.