Why is the gospel named after the Apostle Thomas?

Why is the gospel named after Thomas? 

In v13 Jesus takes Thomas to one side to give him some special and individual teaching. He does this because Thomas is the apostle who demonstrates an enhanced level of understanding regarding who and what Jesus is, superior to both Peter and Matthew. Note also that in the opening prologue Thomas is referred to as the apostle who wrote down the words that Jesus spoke. In this prologue Thomas is actually referred to as   'Didymus Judas Thomas'. 'Thomas' means twin in Aramaic, and 'Didymus' means twin in Greek, so as the author Thomas is twice referenced as the twin. Clearly there is no biological reference here rather it points to the special bond and kinship that Thomas and Jesus enjoyed.