Answer

Fanny Crosby's original hymn, published in 1875, reads: "And opened the life gate that all may go in."

It was apparently the editors of the original Trinity Hymnal (1961) who changed the words to "And opened the life-gate that we may go in." (It is possible, however, that they were using the wording of an earlier hymnal.)

Furthermore, though you do not mention it, the line in verse 2, "The vilest offender who truly believes, that moment from Jesus a pardon receives" was altered to "forgiveness receives."

These changes were clearly marked: "St. 1, line 4; st. 2, line 4, alt." (though the marking does not specifically state "alt. 1961," which would attribute the change specifically to the Trinity Hymnal). The revised Trinity Hymnal (1990, hymn 55) retained the changes. Orthodox Presbyterians singing from these hymnals use the altered words which are jarring to those who are used to Fanny Crosby's original.

Why did the editors make these changes? They clearly move in a Calvinistic direction.

With regard to the first change, they apparently wanted to safeguard the doctrine of limited atonement: the doctrine that, while Christ's death on the cross for sinners was sufficient for all fallen sons of Adam (all the human race with the exception only of himself, the perfect one), it was intended only for the elect (those whom the Father had given to the Son before the foundation of the world, John 17:2, Rev. 17:8).

The second change is a bit perplexing because "pardon" and "forgive" are linguistically related ("pardon" is from the Latin per- [for] + dōnāre, to give).

It is this writer's opinion that the changes were unnecessary. The term "all" can be sung with the sense "all for whom he died"—that is, the elect ("all" as opposed to the one who died, 2 Cor. 5:14). The vilest sinner who truly believes in Jesus Christ does indeed receive pardon, forgiveness of sins and remission, as well as the imputed righteousness of Christ which alone qualifies him for heaven.

At the time of the publication of the original Trinity Hymnal, OPC minister, theologian and musician E. J. Young wrote several articles about the project in The Presbyterian Guardian, explaining the committee's rationale behind the choice and wording of the hymns included. The articles are available at the links below. Scroll through the pdf file to find the page.

Vol. 30, March 1961, p. 55 Our New Hymnal

Vol. 32, March 1963, p. 38 Knowing Our Hymnal