This song is one of the reasons that people used to call U2 a christian band. There are others, like 40 and Still Haven't Found, but this one is pretty straight out. Incidentally, the song 40 takes its lyrics straight from the 40th Psalm.
I try to sing this song, I...I try to stand up But I can't find my feet I try, I try to speak up
I think it is important to notice the negative connotations here. I try, but I can't do it.
But only in you I'm complete
I think the you should be capitalized here, because to me the you refers to 'The Lord' as in the Lord that is referenced through out the rest of the song.
Gloria...in te domine, Gloria...exultate, Gloria...Gloria
This tells me that Gloria is not a female. Gloria here is Latin, and it is like those Gloria in egg shell sees dayo songs that are about the Lord.
Oh Lord, loosen my lips
I take the straight up interpretation here that loosening his lips is what gives him the courage to talk about the Lord. Allow me to speak freely without fear.
I try to sing this song, I...I try to get in but I can't find the door, The door is open, You're standing there, You let me in
There he goes trying again, but he can't do it. Here he is trying to find a door, but it turns out the door is open, the 'You' (Lord) is standing there and He lets him in.
Oh Lord, if I had anything, Anything at all, I'd give it to you, I'd give it to you
If you just took this sentence out of this song, and said it by itself, it would need no interpretation. It stands on its own.
I think it is incredibly evident with the number of times that 'Lord' is mentioned, that this is NOT a girl, It is his God.
Gloria...Gloria...