Significant figures in conversion factors?
I'm not sure where your conversion factor came from as it is incorrect but regarding the correct factor:
Because the inch is defined using the metre, and the gallon defined using the inch, there is an exact conversion between them—1 US gallon is exactly 3.785411784 L.
Thus, like any exact quantity or conversion factor, we treat the gallon to litre conversion factor as having infinite significant figures.
Because we have defined it exactly, there is no implicit uncertainty in the number.
There are other conversion factors that are not exact and do actually have uncertainties, but for most practical purposes, these have far more significant figures than a normal measured quantity.
See this question for more details.