Since there is no one 'comprehensive' source on how "Gentilim" pray, we must infer from the text what Jesus is talking about, and modeling our understanding accordingly.
But thou, when thou prayest, enter into thy closet, and when thou hast
shut thy door, pray to thy Father which is in secret; and thy Father
which seeth in secret shall reward thee openly.
7But when ye pray, use not vain repetitions, as the heathen do: for
they think that they shall be heard for their much speaking.
8 Be not ye therefore like unto them: for your Father knoweth what
things ye have need of, before ye ask him.
9 After this manner therefore pray ye: Our Father which art in heaven,
Hallowed be thy name.
10 Thy kingdom come. Thy will be done in earth, as it is in heaven.
11 Give us this day our daily bread.
12 And forgive us our debts, as we forgive our debtors.
13 And lead us not into temptation, but deliver us from evil: For
thine is the kingdom, and the power, and the glory, for ever. Amen
This passage of Matthew 6:1-18 is talking about the 3 disciplines of a believer: Almsgiving, Praying, and Fasting. Just as they were disciplines in the life of a Jew, so also were they to be disciplines in the life of a believer.
In each admonition, the recurrent theme is:
and thy Father which seeth in secret shall reward thee openly(vss 4,6,18)
Jesus is telling His audience that when you do any of these things, 1)The Father sees you doing them, 2)The Father will reward you for doing them. Therefore, faith must accompany any of these activities. Paul says in Hebrews 11:6,
But without faith it is impossible to please him: for he that cometh
to God must believe that he is, and that he is a rewarder of them that
diligently seek him.
Our reward does not come our "much speaking", just as it doesn't come from what men see, but what God sees.
An OT example of this is found in 1 Kings 18;24-29 when Elijah contends with the prophets of Baal:
And Elijah said unto the prophets of Baal, Choose you one bullock for
yourselves, and dress it first; for ye are many; and call on the name
of your gods, but put no fire under.
26 And they took the bullock which was given them, and they dressed
it, and called on the name of Baal from morning even until noon,
saying, O Baal, hear us. But there was no voice, nor any that
answered. And they leaped upon the altar which was made.
27 And it came to pass at noon, that Elijah mocked them, and said, Cry
aloud: for he is a god; either he is talking, or he is pursuing, or he
is in a journey, or peradventure he sleepeth, and must be awaked.
28 And they cried aloud, and cut themselves after their manner with
knives and lancets, till the blood gushed out upon them.
29 And it came to pass, when midday was past, and they prophesied
until the time of the offering of the evening sacrifice, that there
was neither voice, nor any to answer, nor any that regarded
Their "much speaking" did not produce any result, and after a while Elijah starts ridiculing them, telling them more or less 'he's hard of hearing'.
But you will notice when Elijah prays(vs 37):
Hear me, O LORD, hear me, that this people may know that thou art the
LORD God, and that thou hast turned their heart back again
Then the fire of the LORD fell, and consumed the burnt sacrifice, and
the wood, and the stones, and the dust, and licked up the water that
was in the trench
James 5:17-18 says:
Confess your faults one to another, and pray one for another, that ye
may be healed. The effectual fervent prayer of a righteous man
17 Elias was a man subject to like passions as we are, and he prayed
earnestly that it might not rain: and it rained not on the earth by
the space of three years and six months.
18 And he prayed again, and the heaven gave rain, and the earth
brought forth her fruit.
"Effectual fervent prayer" does not count the amount of speech or number of repetitions used, rather it trusts in being rightly related to a personal God who hears and answers prayer.
Jesus's admonitions are to believers to pray "as if God were listening to every word you say", therefore "much speaking" isn't going to make Him hear you any better, believing will.