Well if you know how to play the songs you're comfortable playing in the key of Db and the key of Gb, all you need to do is this:
First relax, you're probably making things harder than what they really are. Remember if you want to move to the key of Db from the key of C, you just raise everything a half step.
So just start with a song you know that's in the key of Db (not C#...reread the keynote's post on sharp vs. flats that's in the lessons section) and practice playing that song in C, just take everything you're playing in Db a half step lower (and if you want to go to the key of D from Db raise everything a half step), for example:
let's say that you know how to play "Thank You Lord" in the key of Db: (I'll post how you would play it on the piano and since you said keep it simple, I'll keep it simple, so I'll post the basic chords (it's the choir parts)):
Left hand / Right hand (chord symbols)
I: Eb / Bb,Db,Gb (Gb/Eb)
just: F / Ab,Db,F (Db/F)
want: Gb / Gb,Bb,Eb (Ebm/Gb)
to: G / F,Bb,Db (Bb/G)
thank: Ab / F,Ab,Db (Db/Ab)
you: Ab / Eb,Ab,C (Ab)
Lord: Db / F,Ab,Db (Db)
now lower everything a half step and you would have:
I: D / A,C,F
just: E / A,C,E
want: F / F,A,D
to: F# / E,A,C
thank: G / E,G,C
you: G / D,G,B
Lord: C / E,G,C
now you can modulate to Db by playing this: (this is just a simple progression that will take you to the key of Db from C)
(play "-" = (single notes) with both hands )
C-B-Bb
Ab / Eb,Ab,C
Bb /F,Bb,Db
C / A,C,Eb
Db / Ab,Db,F
that Db chord will start you at the beginning of the song in Db.
Sure, remembering the numbers is fine, but you can keep it simple by remebering if you want to play a song a half step higher, you need to move your notes up a half step.
Also make sure that the key of Gb isn't too high for your praise team & your congregration. Because the purpose of a praise team is to energize and encourage the congregation to join in and sing lift up praises to God. So try to keep you praise songs between Ab (the the left of middle c) to Db (a note above middle c), those keys are comfortable for congregations to sing along to.
Hope this helps some.
