The GSP is a freight forwarding program.
It will not ship items that cannot be imported into Canada, even if they can be legally purchased elsewhere.
The first time this came to the attention of the Boards was about guitars made with rare woods, which were stopped under CITES environmental protections.
Lithium batteries and some herbal remedies have also been mentioned.
Did you get your money refunded?
In most cases those items would also have been stopped by CBSA.
The GSP is useful for bulky, high value items that would be stopped by CBSA and import fees charged.
Not for small and low value items.