What should I do with an abandoned gas line in my kitchen floor?
Looking at that, and given the age of the house, I would say it's a bayonet type connector for an old laundry copper.

The 'thumbscrew' on the back is the on/off for the gas supply.

I would strongly suggest getting a Gas Safe engineer in to check if it is still 'live'. If it is, and you are planning to put a unit back over where it is, you could probably get him to cap it a bit lower for you. If you are wanting to reposition your kitchen units completely and this is sticking up in what will be open floor, then you definitely need to get an engineer in. If it's no longer 'live', the engineer can advise whether or not you can just cut it off below the planned finished floor level.

Don't second guess gas - recipe for disaster!