I am using Microchip board SAMA5D27-SOM1

I am building "buildroot" by refering below showed link

(https://microchipdeveloper.com/32mpu:buildroot-custom-project)

I would like to add gcc or compiler to run c program related to libusb in that.How to add gcc or other compiler during building.

I'd call up Microchip's Field Engineers for this one. From Microchip's page:

Extensive Ecosystem:
Free Linux® distributions
MPLAB X Integrated Development Environment
MPLAB Harmony 3
Complete set of C examples for bare metal users
Multiple third-party software and hardware partners

You'll likely have to ask about that last item. Folk I know stick with MicroChip's dev kits and offerings.