benefits of practicing gallops UDU UDU UDU...?
 

Have anyone practiced triplets this way ? (UDU UDU UDU UDU)?

No. But if anything, it'll improve your upstrokes! Just remember to accent the first triplet of each beat.

Incidentally, they're not gallops. Just triplets. Gallops are based on a 16th note time frame.

nope, but it seems like a good exercise for upstrokes. i used to reverse the picking directions on some riffs to practice upstrokes. it isn't something to beat yourself up over if you can't do it though.

I think it could sometimes be useful. For example take "Redneck" at 0:29.

No I didn't, I found the best ways to play triplets are:

1) All downstrokes: ddd ddd ddd ddd (for maximum aggressive sound...if the tempo allows to do it)
2) All alternate picking: dud udu dud udu (this is what allows the highest speed)
3) dud dud dud dud (something in between: allows a higher speed than all downstrokes, but is more aggressive than all alternate picking)

If you meant gallops (on eighth note and two sixteenth notes - pattern), I would play it either all down of if it's too fast: d du d du d du d du

But back to your question: yes, you benefit from practicing the way you desribed: By practicing so many upstokes you train your recoil motion to become stronger & faster, and this translates into faster downstrokes.

Hope this helps

Hm... I never even thought of trying it, I bet it would greatly help alternate picking technique as well.

Indeed, essentially the only benefit of picking these three notes UDU is.. more upstrokes than downstrokes. I'm sure Marty Friedman can make it sound kick ass, however, personally I would just stick to DUD because I prefer the sound.

Also, on a side note... they are gallops, most definitely NOT triplets. Bursts of three notes =/= triplets. Whether it is a series of three sixteenth notes followed by a sixteenth rest or two eighth notes followed by a sixteenth note, the point is, you're playing bursts of three notes.
Three eighth note triplets last precisely as long as two regular eighth notes. Semantics, I know, but.. burst of three =/= triplet! :D

^ it makes perfect sense if the downstroke is on the beat, eg

I have. It's worth doing if you think you'll be doing it. I use pretty strict alternate picking (I'm not good at downpicking and I have no real desire to be), so being able to have punchy gallops even when starting on an upstroke is important. If I do a gallop pattern with straight alternate picking, then I'll end up starting every other gallop with an upstroke.

For the record, gallops and triplets are different. A gallop is a figure featuring an eighth note followed by two sixteenth notes (you'll also commonly see the inverse, two sixteenths followed by an eighth note, which is called a reverse gallop). A triplet is a figure of three evenly spaced notes contained within one beat (so a triplet eighth note will fit into the same amount of space as two eighth notes, or one beat of a 4 count measure).