Wherever the floor plate is placed, whether it is the normal position or not, the nearby cells of the neural tube become neurons. Product made after gastrulation AND floor plate cells AND protein active after gastrulation. The simplest case is that the mRNA and protein from gene ess are made after gastrulation since the protein is not needed until then. There is no indication the gene is expressed earlier. The mRNA here is not likely to be of maternal origin as the protein is used so late in development.
Maternal messages control very early events, prior to gastrulation.
Also the protein is made only by floor plate cells, and these cells don't specialize until after gastrulation. In both cases, the protein will be irreversibly denatured before it can be used.