The word 'Gospel' is never applied to the records of Jesus' life by the Bible its self, rather it seems that the word is used of the message proclaimed, see for example: Matt. 4:23, Matt. 9:35, Matt. 11:5, Matt. 24:14, Matt. 26:13, Mk. 1:1, 14-15, Mk. 13:10, Mk. 14:9, Mk. 16:15, Lk. 4:18, Lk. 7:22, Lk. 9:6, Lk. 20:1, Acts 8:25, Acts 14:7, 21, Acts 15:7, Acts 16:10, Acts 20:24, Rom. 1:1, 9, 15-16, Rom. 2:16, Rom. 10:15-16, Rom. 11:28, Rom. 15:16, 19-20, 29, Rom. 16:25, 1 Co. 1:17, 1 Co. 4:15, 1 Co. 9:12, 14, 16, 18, 1 Co. 15:1, 2 Co. 2:12, 2 Co. 4:3-4, 2 Co. 8:18, 2 Co. 9:13, 2 Co. 10:14, 16, 2 Co. 11:4, 7, Gal. 1:6-9, 11, Gal. 2:2, 5, 7, 14, Gal. 3:8, Gal. 4:13, Eph. 1:13, Eph. 3:6, Eph. 6:15, 19, Phil. 1:5, 7, 12, 17, 27, Phil. 2:22, Phil. 4:3, 15, Col. 1:5, 23, 1 Thess. 1:5, 1 Thess. 2:2, 4, 8-9, 1 Thess. 3:2, 2 Thess. 1:8, 2 Thess. 2:14, 1 Tim. 1:11, 2 Tim. 1:8, 10, 2 Tim. 2:8, Phlm. 1:13, Heb. 4:2, 1 Pet. 1:12, 25, 1 Pet. 4:6, 17, & Rev. 14:6. So Luke did not intend to write a 'Gospel'.
Luke intended to write an orderly and well researched account of the life of Jesus for a man called Theophilus and he opens his account by saying that others have undertaken the same task - what he means by the word 'many' is unclear. R J Dillon for example suggests Luke is referring to "two or three" other records [Previewing Luke’s Project from His Prologue (Luke 1:1–4)," CBQ 43 (1981):207] but then he might be more influenced by his resolution to the synoptic problem.
Personally I just think this is rhetorical preface that Luke employs to begin his account, the NAC reads:
Since “many” and its related expressions are frequently found in
rhetorical prefaces, the term should not be taken as an exact
reference to a specific number. It probably is best to understand it
as meaning others.
[Stein, R. H. (1992). Luke (Vol. 24, p. 63). Nashville: Broadman & Holman Publishers.]
The reference to 'many' as a rhetorical device would be to focus the readers mind on just how important the events Luke is recording are. It is, as if he is saying "look, it is just me that is writing about these events, everyone is at it. So you had betetr natice of what I am saying.'
So, yes other people were writing accounts of the life of Jesus, but these were not 'Gospels' because that name was given to them later and most of extant books that have also been labelled 'gospel' that have been discovered since have not been demonstrated to be written before Luke so, even if Luke was aware of writings that existed then, they have not stood the test of time which suggests the early church did not find then valuable.