In general, MyPoints does not reward for the purchase of GCs. There are a few exceptions such as purchases made at Giftcertificates.com and Tupperware.com.
Also, they will not reward for the use of GCs for purchases. In other words, if you use a GC to partially pay for a purchase, only the amount over and above the value of the GC will be used to calculate your reward. If you use a GC to completely pay for a purchase, you get no reward at all.
Additionally, MyPoints does not reward for any taxes paid nor for shipping and handling charges, only for the amount of the purchase itself.
It would be nice to "double dip" this way but won't work.