I'm sure there are others on this Forum who probably have this same exact setup, but I'll try to give you some pointers from my experience with other gear.
I prefer to try everything else first... Before trying to use an Isolation Transformer or a Line Conditioner.
The first thing I would suggest is to have just the Helix and the Gemini as the only two items plugged into a grounded Power Strip. Unplug any effects, or other devices connected to the Helix except for your guitar. (So you can eliminate any possible ground loops coming from other devices)
Which input of the Gemini 1 are you plugging into, and what cabling are you using?
Gemini 1 SpecificationsI/O Input 1 â€“ Combo XLR-1/4â€³ TRS jack line input (mono or stereo) Input 2 â€“ Combo XLR-1/4â€³ TS jack instrument input
The 1/4" outputs of the Helix are TS and and are switchable between Instrument level and line level. I would suggest you use a high quality shielded 1/4" Instrument cable to connect from the Left output of the Helix (Mono) to Input 2 on the Gemini. In this case you would have the Helix set to Instrument level on the 1/4" output. Of course, this is an unbalanced connection, and it may pickup interference if the cable run is long. (But if everything is setup properly it should be perfectly usable in most cases)
For a long cable run I would recommend using the Left XLR output of the Helix (Mono) into Input 1 of the Gemini. Use a high quality Microphone cable and set the XLR output of the helix to Line level. This balanced cabling should eliminate the majority of Buzz and hum.
Of course, there are situations where the "Building Ground" is poor on your power distribution in a particular venue and you need to look to other options, such as a Power Conditioner, but hopefully one of the items above will be the ticket...