It was faith that was preached to Abraham before the gospel (was preached to anyone). That is reinforced in the Greek which used the word
προευαγγελίζομαι [proeuaggelizomai] meaning "to preach before the Gospel or to preach in advance of the Gospel". It is also reinforced by what follows, namely the entire discussion of verses 9-14 concerning that the righteous shall live by faith.
It was the Gospel which was preached to Abraham Gal 3:8
8 And the scripture, foreseeing that God would justify the heathen through faith,
preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
Also the Gospel Paul preached was according to the OT scriptures 1 Cor 15:3-4
Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
3 For I delivered unto you first of all that which I also received, how that
Christ died for our sins according to the scriptures;
4 And that he was buried, and that he rose again the third day
according to the scriptures:
Isaiah was scripture Isa 53:5-6,10-11
5 But he was wounded for our transgression
s, he was bruised for our iniquities: the chastisement of our peace was upon him; and with his stripes we are healed.
6 All we like sheep have gone astray; we have turned every one to his own way;
and the Lord hath laid on him the iniquity of us all.
10 Yet it pleased the Lord to bruise him; he hath put him to grief:
when thou shalt make his soul an offering for sin, he shall see his seed, he shall prolong his days, and the pleasure of the Lord shall prosper in his hand.
11 He shall see of the travail of his soul, and shall be satisfied:
by his knowledge shall my righteous servant justify many; for he shall bear their iniquities.
Paul preached and believed the Same Gospel of Christ as Isaiah did.