Yes. Aluminum.
ALSO, see this
pic from 6066 GMCCLUB/Jolly pages/Hub Caps/Arthur Throckmorton.
The 62-63 era was evidently a transition period at GMC regarding hub cap style. (Nubs being phased-in for clips.) The size of the Ebay pair look to be larger than the 1/2T shown in the pic from our site. In the pic from 6066, left is nub cap, right is clip cap. I assume you have a 1/2T. Buyer beware.