How to use GPU to speed up the assignment process?

For example, how to speed up the following program using GPU or removing the loop? (At fact, the actual number of variables x and z is more than 10^6. but to convenience, I just list 16 values ).

clear;

tic;

x=[1,2,3,4,5,6,7,8,10,12,13,14,15,16,17,18];

z=[1,125,612,365,489,612,365,855,1,126,610,365,489,610,365,859];

k=zeros(length(x),121*121);

for i=1:1:length(x)

k(i,z(i))=x(i);

end

k1=sum(k,1);

toc;