Neither Gregorian Chant NOR polyphony are required to make a Mass valid. If a Parish likes guitar Masses, the participants at Mass are singing, praising God, receiving the Eucharist respectfully, why should that be an affront to anyone, or considered not valid? YOU or others may not like it, but there are usually several other Masses available at most Parishes on a weekend, so you don't have to attend a Mass with music that does not support your idea of worship.
I used to play Folk Masses all the time. I like some of the guitar Masses nowadays, but I prefer the more traditional music played on the organ and other classical instruments. That music doesn't make the Mass BETTER, just different. The music makes me more attuned to the meaning of the Mass, but it may be just the opposite for folks who prefer guitars and drums at Mass. They may find that they are more inclined to contemplate God and the awesome Gift he sent to us, when they're singing an upbeat hymn.
Just like STM, you should spend a number of years studying the writings of the Pontiffs on the topic.
Not the Weaklands, Bugninis, nor Pilarczyks.
In other words, seek the truth, not the "tongues which tickle your ears."
That's a word to the wise, if you can handle it.