If I play chords on my guitar to produce nice long pad sounds, it works fine, but inevitably there is a small gap as I move from (say) an F chord to a C chord. The "problem" can be reduced by increasing the decay time on the patch, but what I usually end up doing is diving into the piano roll and extending all the Midi notes manually, so that the notes comprising the F chord run right up to the start of the C chord.

Is there a way to do this less labour intensively?

A longer release time on the synth's amplitude envelope so the notes sustain a little after the MIDI note itself ends might do the trick.

I have a hold pedal on my gr20.

You may want to check out in the September issue of Sound On Sound a review of Jam Origin Midi Guitar 2. This software converts the guitar signal into midi data.  But it only uses the standard output from your guitar.  No special pickups necessary or anything like that.  Hence it is converting your polyphonic playing!
 
But here is the thing, the reviewer stated quite clearly this beats the pants off any hex pickup going into anything previously.  For speed and accuracy.  Hard to believe I agree.  It is around 83 pounds. ($112 US) 
 
He said he is never going back to any previous system.  This might be worth a look.  Can be used stand alone or inside your DAW too. 
 
They have never stopped developing guitar playing to midi conversion and it seems it is just getting better all the time. 

If the pad is not the instrument in focus, copy/paste can be your friend since most chord progressions repeat to some level. You can also drag/drop into the browser as you work if desired, so will start building a chord library as you go that can be drag/dropped from the browser (which is a rudimentary chord track generator that is easy to modify if you save one clip per chord).

Depending on various factors quantizing note duration might do the trick.  
Fast, easy and can be applied to multiple bars.
 

Jeff Evans
You may want to check out in the September issue of Sound On Sound a review of Jam Origin Midi Guitar 2. This software converts the guitar signal into midi data.  But it only uses the standard output from your guitar.  No special pickups necessary or anything like that.  Hence it is converting your polyphonic playing!
 
But here is the thing, the reviewer stated quite clearly this beats the pants off any hex pickup going into anything previously.  For speed and accuracy.  Hard to believe I agree.  It is around 83 pounds. ($112 US) 
 
He said he is never going back to any previous system.  This might be worth a look.  Can be used stand alone or inside your DAW too. 
 
They have never stopped developing guitar playing to midi conversion and it seems it is just getting better all the time. 

Hi Jeff

Meant to say, I am actually using Midi Guitar 2! It's absolutely brilliant, I agree. It was Paul White's review in SOS that persuaded me to get it.

I'm interested in this for a bunch of reasons, I'd been thinking of looking into either getting a midi pickup set up or upgrading Melodyne for the version that does polyphonic converting. Mostly I just want to be able to play a guitar DI track for pads and use Dimension, Rapture etc for the sounds. I've also heard of using melodyne to keep bass guitar notes in tune, which is an interesting thought. The only other thing I could think of using guitar to midi for is maybe taking a guitar DI and maybe using a plug in to layer a strat sound on top of my LP track.

stm113cw
I'm interested in this for a bunch of reasons, I'd been thinking of looking into either getting a midi pickup set up or upgrading Melodyne for the version that does polyphonic converting. Mostly I just want to be able to play a guitar DI track for pads and use Dimension, Rapture etc for the sounds. I've also heard of using melodyne to keep bass guitar notes in tune, which is an interesting thought. The only other thing I could think of using guitar to midi for is maybe taking a guitar DI and maybe using a plug in to layer a strat sound on top of my LP track.

I'm not saying it's perfect but if you listen to the top track on my SoundCloud, Alicante in September, (link below), the "choir" was played by my Strat using MIDI guitar 2.

When using Midi Guitar, a good thing to have as far as string dampeners are concerned is a small strip of velcro.....like in this video.