Let's say I have a 1" diameter 1018 Steel Rod Coupled to a nut made of gold. If the setup is kept in a constantly replenishing salt water bath (3% NaCl), is there a way to calculate the theoretical mass loss of the steel rod after a given amount of time?

Put the two pieces in the solution uncoupled but close and measure the current flow. Amps (actually nanoamps) are electrons and they will be exchanged for metal ions. Basic electrochemistry.

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Plymouth Tube

Can I just use a galvanic series to approximate this current ?

use farady's law for corrosion,

If you want theoretical not actual measurements then you need the polarisation curves for gold and steel in 3M NaCL which give current per square centimetre (specific current) and voltage- where they cross gives the specific current then times by surface areas to get real currents and use Faradays's Law to get metal loss. Since the bath is constantly refreshing the curves would be fine as given- if not refreshed then the solution will change to iron chloride and sodium chloride and no longer be valid. I assume constant aeration.