The Gospel of the Ebionites is known only by the quotations from Epiphanius in these passages of his Panarion: 30.13.1-8, 30.14.5, 30.16.4-5, and 30.22.4.
All our knowledge of this is derived from Epiphanius, and he uses very confusing language about it (as about many other things). The passages are as follows:
And they (the Ebionites) receive the Gospel according to Matthew. For this they too, like the followers of Cerinthus and Merinthus, use to the exclusion of others. And they call it according to the Hebrews, as the truth is, that Matthew alone of New Testament writers made his exposition and preaching of the Gospel in Hebrew and in Hebrew letters.
Epiphanius goes on to say that he had heard of Hebrew versions of John and Acts kept privately in the treasuries (Geniza?) at Tiberias, and continues:
In the Gospel they have, called according to Matthew, but not wholly complete, but falsified and mutilated (they call it the Hebrew Gospel), it is contained that 'There was a certain man named Jesus, and he was about thirty years old, who chose us. And coming unto Capernaum he entered into the house of Simon who was surnamed Peter, and opened his mouth and said: As I passed by the lake of Tiberias, I chose John and James the sons of Zebedee, and Simon and Andrew and Thaddaeus and Simon the Zealot and Judas the Iscariot: and thee, Matthew, as thou satest as the receipt of custom I called, and thou followedst me. You therefore I will to be twelve apostles for a testimony unto (of) Israel.
And:
John was baptizing, and there went out unto him Pharisees and were baptized, and all Jerusalem. And John had raiment of camel's hair and a leathern girdle about his loins: and his meat (it saith) was wild honey, whereof the taste is the taste of manna, as a cake dipped in oil. That, forsooth, they may pervert the word of truth into a lie and for locusts put a cake dipped in honey (sic).
These Ebionites were vegetarians and objected to the idea of eating locusts. A locust in Greek is akris, and the word they used for cake is enkris, so the change is slight. We shall meet with this tendency again.