According to Wikipedia Q source is sometimes called the Q Gospel. My understanding of Q though is that it is proposed mostly as a collection of sayings. Is it proper to call it a "Gospel"? Why or why not?
The Gospel of Thomas consists mostly of sayings, and it explicitly claims to be a Gospel in its first sentence. So that's pretty good evidence that something like Q would have been thought of as a gospel around the time when Thomas was written (which is sometime between the mid 1st and mid 2nd century, we don't know).
On the other hand, we don't know when "gospel" was first used as a name of a genre or writing. One theory is that it was first used to refer to a genre of writing in Mark 1:1. So it might be anachronistic to refer to Q as a gospel, since the genre may not have yet existed.
To me, in order for any book to be considered a Gospel the first thing it would have to have is the teaching of the death, burial, and resurrection of Jesus Christ since that's what the word means in the underlying NT Greek in the canonical Gospels. That is the good news after all; that Christ died, was buried, and rose again!
Outline of Biblical Usage
1) a reward for good tidings
2) good tidings
a) the glad tidings of the kingdom of God soon to be set up, and subsequently also of Jesus the Messiah, the founder of this kingdom. After the death of Christ, the term comprises also the preaching of (concerning) Jesus Christ as having suffered death on the cross to procure eternal salvation for the men in the kingdom of God, but as restored to life and exalted to the right hand of God in heaven, thence to return in majesty to consummate the kingdom of God
b) the glad tidings of salvation through Christ
c) the proclamation of the grace of God manifest and pledged in Christ
d) the gospel
e) as the messianic rank of Jesus was proved by his words, his deeds, and his death, the narrative of the sayings, deeds, and death of Jesus Christ came to be called the gospel or glad tidings
Note: Emphasis is mine.