It's good to be aware of the difference between "galloping triplets" and true triplets.
"Galloping triplets" are actually eighth-sixteenth-sixteenth groupings.
True triplets are three notes that have the same value.
If they're eighth-note triplets, then all three are played evenly spaced out for the duration of one quarter note.
I like to use Steve Harris as an example of both. There are many Iron Maiden songs in which which Steve uses the "gallop", and a few in which he uses true triplets.
Phantom Of The Opera uses true triplets, whereas The Trooper is galloping 8th-16th-16th groups.
Phantom Of The Opera (Youtube) The Trooper (Youtube)
As for songs which shift back & forth between triplets and quadruplets, I'm not quite sure. But, I've heard drummers do it more often.