If you want to shoot colour, try Portra 400, ISO 400 (or greater) speed, but grain more like a ISO 100 film.
The leaf shutter in the GA645 makes 1/30 fairly easy to pull off as long as your technique is solid - one of the reasons I love that camera :)
A thirtieth is on the slow side; it's certainly doable with practice and good technique but I'd use a tripod at that speed if I wanted to be sure of good negatives. I try to stay in the 1/125 and up range, and that's with any camera handheld.
I can sometimes shoot "acceptable" images at 1/30th with extreme care (leaning on a post or something) but do expect them to be perfectly sharp. But for critical sharpness or when I expect to be doing some enlarging I try not to ever got below 1/125th regardless the focal length and follow the old 1/focal length for lens longer than 100mm.
About 10 years ago at a multi-day photo workshop there were a number of participants boasting about hand holding down to 1/30 or even 1/15. Their chromes on the light table later in the week proved otherwise.
Recently I've been using a tripod for all my shots.
I don't know, that is why I am asking about shutter speed. I think of the GA645 as a 35mm lens camera, so tend to shoot with slow shutter speeds. However, the lens is in reality a 65mm one so I might be over doing it with my slowness. Here are a few examples. The monochrome image has light leak as the roll did not wind properly for some reason. But you can see that nothing is in focus. I dont have the shutter speeds used for these in front of me right now.
Try setting your camera to shutter speed priority and work with various shutter speeds to see at which settings you get reliably sharp pictures.
This one worries me. SHot in very bright sunlight, yet nothing is sharp. Bad s**n?