In my experience, and since this is an acoustic(unlike your ES-125), I'd suggest light(12-54) or (for myself) medium gauge phospher bronze(13-56). Acoustic archtops usually benefit sound-wise from lights or mediums, and from bronze strings. That's what I use on my various vintage Epiphone acoustic archtops. Brand is not so important: Martin or D'Addario are fine.
Amplifying an acoustic archtop can be frustrating: the piezos, like K&K brand, that work pretty well in a flattop, do not(IME) sound great in an acoustic archtop. The one made by Pick Up The World, which is a strip that is sandwiched under the bridge base, might be ok, with good EQ. Usually I try to use a microphone, when playing out. I did recently mount a Kent Armstrong floating 'Johnny Smith' type pickup(salvaged off a damaged Eastman) onto my '43 Epiphone Ritz, and it sounds pretty good, for that Kenny Burrell/Tiny Grimes sound. The KA mounts to a pickguard. That's of course a different sound, than the guitar through a microphone.
Hope that helps- nice old Gibson you have there! and with a vintage HSC!