The number is added by default if you just use the high numbers directly
x = [10:0.1:12]*10^9;
y = sin(x);
plot(x,y)
-- EDIT --
If your x values range from 10 to 11.5 then you can simply multiply them for every plot you make:
x = [10:0.1:12]; % old x values from 10 to 12
y = sin(4*x);
x_new = x*10^9; % multiply with 10^9 to scale data accordingly. From 10e9 to 12e9
figure(1);
plot(x_new, y)
xlabel('Frequency (Hz)')
If you really want to use custom ticks, check out this link to the documentation. You could use the last tick to display, that the values are meant to be times 10^9
figure(2);
plot(x, y)
xlabel('Frequency (Hz)')
xticks([10 10.5 11 11.5 12])
xticklabels({'10', '10.5', '11', '11.5', 'x10^9'})
-- EDIT 2 --
Or in your case you could just edit the x-label to say "Frequency (GHz)". Since Gigaherz is x10^9 Hz
