In order to use GPAW on Seawulf, you must load both a module and an Anaconda 3 environment:

module load gpaw/1.5.1
module load anaconda/3
source activate gpaw

And to remove GPAW from your environment:

conda deactivate
module unload gpaw/1.5.1

Note: it is possible to corrupt your environment if you load/unload modules and activate/deactivate environments in the wrong order because Anaconda environments and modules are not perfectly compatible. To avoid running into issues, load modules before loading Anaconda environments (this is true in general, not just for GPAW).