I've been through the same kind of frustration with various modeller's and other amp's etc. I hate to say this but sometimes the best you can hope for is a compromise, or you could drive yourself mad trying to dial in a tone that just isn't there. I've never used a GDec, so I'm not saying it can't do it, but it sounds like you've tried a lot and haven't managed.
You have to assume that the people at Fender didn't just dream that "British Jangle" tone out of nowhere, so there must be some element of an AC30 in there, so it's probably true "Jangle" will get you closest.
Some things to try (since it's a modelling amp) is:-
1) Turn off all other effects, including reverb.
2) Use a guitar that closest matches one of Edge's - I see you got a present from Santa
.
3) Apologise to your family/flatmates/neighbours, then turn drive down/off (if it has one) and crank the volume up as loud as you dare (on the Jangly setting). Set bass/mid/treble to 0. It will sound AWFUL... but be patient. Turn up Bass bit by bit, and stop when it sounds too "Boomy/Farty/Flappy". Turn up Treble bit by bit and stop when it sounds too harsh/bright/unpleasant that it hurts your ears, then turn it down 1-2 notches. Now turn up mid until it fills out the sound and sounds pleasant with Bass and Treble as you set them above.
4) Following 3 you should now have at least a pleasant tone, because your ears tuned it in, even if it isn't exactly AC30 or Edge.
5) Carry on playing and make slight adjustments to treble and mid to try and round off the high frequencies.
6) Repeat the process every time you play - just to start tuning your ears. You'll zone in to a U2-like tone without realising.
The reason to turn up your amp full is because the amp and speaker are designed as a pair. 30 watts can be very loud. So while you're at bedroom playing level, you might not be driving the speaker hard enough to hear the AC30 tone, it won't be resonating the cone hard enough to bring out all the character that the AC30 amp sim is providing. Another tip when setting the tone is play your guitar across the fret board and listen. I usually play - An Open E Chord (all 6 strings), Barred A Chord 5th Fret, Barred D Chord 10th Fret and then some double stops (2 string chords) up above the 12th fret (try chorus part out of Beautiful Day). This tests the amp response across the range of your guitar. Make sure Guitar Volume and tone are on FULL. The Open E chord will tell you if your Bass control is too boomy, The double stops will tell you if your Treble is too bright. Then everything in between is exactly that - in between.