gnuplot> pwd  
C:\Users\n0ahb\Documents
gnuplot> r1 = 1e9
gnuplot> plot "Lab1Data.txt" using ($1*r1):2 w l
gnuplot> set ylabel "Light(V)" 
gnuplot> set xlabel "Time (nS)" 
gnuplot> unset key
gnuplot> plot "Lab1Data.txt" using ($1*r1):2 w l
gnuplot> f1(x) = a1*exp(-(x-x1)*(x-x1)/2/d1/d1) 
gnuplot> a1=0.35 
gnuplot> x1=1.35
gnuplot> d1=1  
gnuplot> plot [-5:8] "Lab1Data.txt" using ($1*r1):2 w l  
gnuplot> plot [-5:8] "Lab1Data.txt" using ($1*r1):2 w l, "Lab1Data.txt" using (r1*$1):(f1(r1*$1)) w l lw 3  
gnuplot> fit [-5:8] f1(x) "Lab1Data.txt" using ($1*r1):2 via x1, d1,a1   
                                  
...

After 4 iterations the fit converged.
Final set of parameters            Asymptotic Standard Error
=======================            ==========================
x1              = 1.48714          +/- 0.008616     (0.5793%)
d1              = 1.08225          +/- 0.008616     (0.7961%)
a1              = 0.349307         +/- 0.002408     (0.6893%)
...
gnuplot> plot [-5:8] "Lab1Data.txt" using ($1*r1):2 w l, "Lab1Data.txt" using (r1*$1):(f1(r1*$1)) w l lw 3
gnuplot> plot [7:18] "Lab1Data.txt" using ($1*r1):2     
gnuplot> x1=11.5 
gnuplot> fit [7:18] f1(x) "Lab1Data.txt" using ($1*r1):2 via x1, d1,a1   
                                 
... 
After 5 iterations the fit converged.
Final set of parameters            Asymptotic Standard Error
=======================            ==========================
x1              = 11.3436          +/- 0.01208      (0.1065%)
d1              = 1.09553          +/- 0.01209      (1.104%)
a1              = 0.165918         +/- 0.001594     (0.9609%)

gnuplot> plot [7:18] "Lab1Data.txt" using ($1*r1):2 w l, "Lab1Data.txt" using (r1*$1):(f1(r1*$1)) w l lw 3