It's a really simple bass run. It's in the key of G, but this is a good song to learn for any key. I'm going to give the numbers so that it can be learned in any key, then I'll lay out the notes for the numbers in G.
Chorus:
1 5 6 1 5 b5
I've got a key to the kingdom,4 5 6 1 5 6
I've got a key to the kingdom,1 5 6 1 3 4 #4
I've got a key to the kingdom, and the5 5 1 3 4 #4 5 6 #6 7
World can't do me no harm, *turnaround bass line they use sometimes
Verse (words change, but the format remains the same):
1 5 6 1
Well I woke up early this morning,5 6 1 4 #4 5
And I got down on the ground, 1 4
I prayed to God in Heaven above, 5 1
And the Holy Ghost came down,_______________________________________
____________________________________
The key of G numbers are:
1 = G
#1/b2 = G#/Ab
2 = A
#2/b3 = A#/Bb
3 = B
4 = C
#4/b5 = C#/Db
5 = D
#5/b6 = D#/Eb
6 = E
#6/b7 = E#/F
7 = F#
The bass line in this song uses the quartet and shout bass lines that's been in church for years. The quartet bass line is basically 1, 3, 5, 6, (repeat forever). Also, here's a link with the shouting bass lines in every key to help you play these types of songs:
http://www.learngospelmusic.com/forums/index.php/topic,84773.msg940912.html#msg940912