From what I understand, the GIP is reduced by the factored pension; so in this case, your person will just receive their £13 392 pension, and no GIP. However, to make things more complicated, as a GIP is tax free, I think an amount of the £13 392 IP equal to the GIP calculation will be paid tax free, if you see what I mean.
So £13 392 - £8 864.97 = £4 527.03, so £4 527.03 of the pension will be taxable income, the remaining £8 864.97 will be tax free.
Disclaimer; I looked a while ago for a colleague and never more than the gov.uk webpages, but that was my understanding!