I am not familiar with gnuplot but from a quick look at the doc, it seems that if you saved a script containing the data and its error bars plus the fit model (or one file for the data and one file for the fit model), calling gnuplot with that (or those) file(s) as argument to a System Exec.vi call, you should be able to get what you need in a result file generated by the script.
Since there would probably be no way to get a call back from gnuplot telling your calling VI that the job is done, you would have to design a wait loop checking for the creation of the result file, which wou would then have to read.
