Is there any historical evidence of Sabbath keeping by Gentiles during the first centuries of Christianity? Did Gentiles use to keep the Sabbath (sunset Friday to sunset Saturday) during the first centuries of Christianity? How widespread was the practice in comparison with Sunday meetings? What does the historical evidence say about all this?
The question of whether Gentiles had to follow the Law of Moses surfaced right at the beginning of the church.  This is to say that some Gentiles were in fact following the Law, including keeping the Sabbath (sunset Friday to sunset Saturday).

And being brought on their [Barnabas, Paul, and others] way by the church, they passed through Phenice and Samaria, declaring the conversion of the Gentiles: and they caused great joy unto all the brethren.
And when they were come to Jerusalem, they were received of the church, and of the apostles and elders, and they declared all things that God had done with them.
But there rose up certain of the sect of the Pharisees which believed, saying, That it was needful to circumcise them, and to command them to keep the law of Moses.
Acts 15:3-5

The conclusion was this.

For it seemed good to the Holy Ghost, and to us, to lay upon you no greater burden than these necessary things;
That ye abstain from meats offered to idols, and from blood, and from things strangled, and from fornication: from which if ye keep yourselves, ye shall do well. Fare ye well.
Acts 15:28-29

So, there was an attempt by certain ones to enforce the Mosaic Law onto the Gentiles and no doubt they were trying to follow the Law, but the Jerusalem Council decided it was not necessary.
Subsequent to that episode, this conflict about the relationship of the Law and grace continued to fester.  Here is Philip Schaff on Eusebius' comment about the Ebionites.

The Ebionites were not originally heretics. Their characteristic was the more or less strict insistence upon the observance of the Jewish law; a matter of cultus, therefore, not of theology, separated them from Gentile Christians. Among the early Jewish Christians existed all shades of opinion, in regard to the relation of the law and the Gospel, from the freest recognition of the uncircumcised Gentile Christian to the bitterest insistence upon the necessity for salvation of full observance of the Jewish law by Gentile as well as by Jewish Christians.
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In sum, there is historical evidence of Sabbath keeping by Christian Gentiles.