Has anyone tried using a cable gland glued into a dry bag for waterproofing underground electronics? Does it work?

No - but I have got a cable into a drysuit (waterproofly) by scrounging an old drysuit inflator fitting, removing the button and spring and potting the cable in through it. Worked fine for powering an electrically heated undersuit and never leaked.

Pitlamp thats interesting what did you use to pot it with? epoxy resin? I am esentially doing the same thing the cable is for a heating pad i have problems with the grommets because the jacks for the heat pad are angled so i cant pass them through the gromet if i can find something lower profile i can get the jack around and then pot it in that might kill two birds with one stone.

I've used epoxy resin, which is fine as long as you securely tape the bottom to stop it slowly flowing out again. But I've also used that 2 part stuff which is sold for mending dents in car bodywork (the stuff that you smear on, let it set, then sand down) which also worked perfectly.

Drysuit inflators come apart easily; the button and spring are usually retained with a circlip that can be prised off with standard caving hostel tools e.g. a couple of forks). They have plastic housings which wear in gritty sumps and eventually allow air to dribble into the suit. The only remedy is a new replacement. Try asking around folk who have been cave diving for a lot of years; they often have one or two marginal ones in the spares box. Or go to any dive shop which does maintenance work on drysuits and they often have something kicking about which you might be able to scrounge.

Its worth pointing out the inflators that Pitlamp is talking about have a big rubber flange that acts as a seal. They do make a proper pass-through for EO cords it's not cheap though.

The issue you will have with a cable grommet is its size it's normally just a 2mm o ring and the hole needs to be the exact right size to seal it properly which is hard in the material.

No idea what sort of size you're after, but if you're just wanting a seal around a hole that you're going to fill with epoxy, a small plumbing tank connector could work? I've used them on expeditions to attach a hose and tap to the bottom of a bag, but you could just run your cable through one and pot it? It would get around the issue of cable grommet sizes that Tseralo mentioned above.

Light Monkey (formerly Salvo) in Florida make a dedicated bulkhead connector for this. Sits flat against the suit/bag. Standard E/O connectors on the wet side. ?120(!) with the cables.

https://deep-ideas.co.uk/Store/index.php?main_page=product_info&cPath=62_44&products_id=360&zenid=6a0b55623d192ed53f7b64fa23234b53

This version is separate from the inflate valve.