Hi @cool200140 and welcome to the community!
Technically, yes, you can ‘hack’ your gamerscore without an Xbox 360, but there’s a hitch. You need to have your profile on a USB flash drive formatted in FAT32.
Once you do that, you can use the profile modding tools available in Horizon. For reference, here’s a thread that describes all Horizon features in detail: Horizon | The Basics.
The only thing about not using a 360 is that you can’t see if what you’ve done actually worked, but you can always borrow a friend’s 360 or grab one cheap online.
I hope this answers your question and/or helps you out 