A "healthy" quantity of RAM for Ignition yields a sawtooth pattern on the gateway's memory trend (gateway status => Performance) where the regular drop-offs end up somewhat below 50% and the GC event at each peak, typically around 90%, doesn't create a "Clock Drift" warning.
If your trend already looks like this, or better, then more RAM will not help your CPU utilization. If your memory trend is not dropping below 50% after the large peaks, then you might be helped by more RAM.
Show a screenshot of your Performance page.
Also, if your gateway is running anything other than Ignition (like your DB), you should migrate those services to other hardware.