Q: I’m sure that the Gloria has been forbidden as an entrance hymn, but can’t remember where.
A: I don’t know of something explicitly forbidding the use of the Gloria as an entrance hymn, but that is what is implied by its position in the Order of Mass.
When the revisions of the Order of Mass were underway in the late 1960s, the Consilium’s study group considered moving the Gloria to a position after the collect or even after the prayer after communion. I mention this in my book At the Supper of the Lamb.
By leaving it in place just before the collect, the members concluded that that’s where it belongs.
It’s a little like asking for a document forbidding the use of the Lamb of God as the entrance hymn. I don’t know of one, but that’s not the function of the Lamb of God, and its position in the Order of Mass presumes its exclusion at other times.