Producer Mitch Easter's Gemini II was used on Murmur, according to these five sources.

R.E.M.'s Murmur by J. Niimi (2005)

Peter Buck's workhorse amp—his Fender Twin—was broken at the time, so Easter loaned him his checkerboard-grill Ampeg Gemini II for the session, which was used on most tracks, alongside the studio's little solid state (i.e., transistor rather than tube-driven) Kasino amp. Guitar-wise, Buck had brought his maple-glo Rickenbacker 360, which he had also used on the Chronic Town session.

RickResource, blue330 (Mitch Easter), April 14, 2008 comment on the thread "What about Peter Buck?"

Mr. Buck played a 360 on Murmur, not a 330. And I recall it was the Jetglo one, not the Mapleglo one he had on their first session here in NC. On Murmur, the amps were a little Kasino solid state thing (Pilgrimage only), and an Ampeg Gemini II. No AC30s! They came much later, and the last couple of times I saw them his amp was a small Silvertone.

Mitch Easter

Sound on Sound, November 2009, “REM ‘Radio Free Europe’ | Classic Tracks”

Standing on the left side of the studio, Peter Buck used Easter's own Ampeg, wide open without gobos and miked with an EV 635 or, for some overdubs, a compressed Neumann U47 FET.

Music Radar, June 3, 2021, “Producer Mitch Easter shares the inside story of R.E.M.’s early recording sessions: ‘It was glorious. They rehearsed a lot just because they liked to play together’”

Tell us about the guitar sounds…

“Peter [Buck] played all the guitars. I think he had a [Fender] Twin Reverb then. He used those for a long time. In the session, he played an electric 12-string, but not a Rickenbacker. It was a Fender Electric XII. I didn’t have a lot of recording equipment, but I did have a good guitar and amp selection.

“I told him, ‘You might want to try this.’ When we did Murmur [in 1983], I remember Peter’s Twin Reverb was dead, so that record was done with my 60s Ampeg Gemini II. It’s on every song on Murmur except Pilgrimage. I think the guitars on Murmur are the classic early Peter Buck sound.

“The thing about Peter Buck’s sound that’s really important is he used larger gauge flatwound strings. That’s what made it all work. Those Rickenbacker pickups sound better the more metal you put over them. And you develop a very deliberate way of playing. That’s what you can hear in his playing. You can hear everything he did. He really mastered it.”

[Guitar Player, February 9, 2022, "'A Lot of Bands Simply Wanted to Fit In, but R.E.M. Always Wanted to Be Themselves': Producer Mitch Easter Reveals the Genius Behind the Jangle Pop"]

What amps was Peter using at first?

He had a Fender Twin, and that’s what we recorded with. I remember when we were doing “Pilgrimage” at Reflection Studio in Charlotte, the Twin was in the shop. [“Pilgrimage” was recorded prior to Murmur as a “test” for I.R.S. Records.]

I didn’t have anything with me, but there was a Kasino solid-state amp in the studio, so we used that, and it sounded great. I think some people might faint knowing that we used this little 25-watt solid-state practice amp, but it had a really nice sound. There were oddball things like that.

For Murmur, he still didn’t have his amp back, so he used my Ampeg G-15. That amp and the Kasino are what’s on Murmur. When they came in to do Reckoning, he had his Twin back. But if you’re a distinctive-sounding guy like him, you sound like yourself no matter what.

There is a photo of Wes during a recording session in which an Ampeg Gemini mic'd up.

JD uses a Gemini I and II.

From this video in 2013 and seen in multiple live images.