There are two libraries of XC functionals that can be used in GPAW. These are libxc and libvdwxc. Conda installation of GPAW automatically picks them. You can check whether your GPAW connects to libxc and libvdwxc like gpaw info.

libvdwxc is useful when you wish to run calculations with vdW-functionals and GPAW. Such as BEEF-vdW. Herewith, libvdwxc implementation of vdW-functionals are better parallelized than the native GPAW implementation. For example, add the following line to your GPAW calculator xc={'name':'BEEF-vdW','backend':'libvdwxc'} to run a calculation with the BEEF-vdW functional. BEEF-vdW calculations with libvdwxc can run as fast as PBE-like calculations if you use the proper grid, like parallel={'augment_grids':True,'sl_auto':True}. Here is a list of libvdwxc functionals: gitlab.com/libvdwxc/libvdwxc

Note that the following GPAW page is somewhat outdated:
wiki.fysik.dtu.dk/gpaw/documentation/xc/vdw.html

libxc is useful when you wish to run calculations with functionals that are not implemented in GPAW. Note that GPAW implementation is more efficient. There are many ways to call for libxc. For example, add the following line to your GPAW calculator xc='MGGA_X_SCAN+MGGA_C_SCAN' to run a calculation with the SCAN functional. Nore that GPAW setups are for LDA, PBE, and RPBE. You can generate setups specifically for your functional if it is GGA or HGGA. Here is a list of libxc functionals: tddft.org/programs/libxc/functionals/