Is a gametophyte a spore?
No.
But close.  Very close.  Basically, a gametophyte is a spore that has / is undergoing mitotic division.  Therefore, it is still haploid, having not undergone fertilization yet, but it is no longer a dormant, unicellular spore; it's passed into the new phase of gametophyte.
The gametophyte produces sexual organs which produce the gametes that will actually be used in fertilization.  Two gametes fuse, at which point, a diploid cell is produced (called a zygote, just as in animals).

The meiospores are the spores you see in the picture on the far left: they are dormant, unicellular, haploid spores, which will turn into the gametophytes.  (meio + spores; spores that have undergone meiosis)

Sources:

Meiospore, Biology Online
Gametophyte, Wikipedia
Plant Life Cycles, Developmental Biology, 6th ed.
Spore, Wikipedia