First, not sure why your thread was marked as spam (immediately, by the system). I un-marked it to teach it that it was wrong, but still deleted it because now it's duplicate. Sorry about that
Second, there's no clear yes or no answer, as it's a bit more differentiated. Note that "proc" is the wrong word to use here; a proc is something else. But anyways, Davlok has the answer:
Madstone with MR: (no Gungdo)
Target without EP: Madstone applies a fresh non-MR EP and detonates it.
Target with EP: Madstone does NOT apply a new EP (refreshes the duration) and instantly detonates that original EP. All subsequent SSS strikes apply a non-MR-EP and detonates immediately as expected. If the original EP was MR-buffed, only that single EP detonation would occur and the rest would be "plain" EPs.
Madstone with MR: (with Gungdo)
Target without EP: Madstone applies a fresh non-MR EP and detonates it. That first EP is then "refreshed" by Gungdo, then "refreshed" by Madstone on the next strike, and then detonated again for the rest of the SSS.
Target with MR-buffed-EP: Madstone does NOT apply a new EP (refreshes the duration) and instantly detonates that original EP. Gungdo then refreshes the MR-buffed-EP on the original target and Madstone keeps refreshing the EP Gungdo just re-applied. So all explosions are MR-buffed, and Madstone is really doing nothing useful in this case.