What is the alternative to gas heating in a 30-year-old 4-unit house?

You should find yourself a good partner. There are engineering firms that calculate heating loads, heating systems, and can show solutions. If nothing has been done for 30 years, the facade and the roof are slowly due anyway. Maybe also the windows and basement ceiling insulation, then a heat pump with radiators will work.

You are in the comfortable situation that the heating systems are still running and currently gas is still available. So you have no stress now, you can "conveniently" prepare everything and then replace it when it fits.

And no - from a long-term perspective, there is no alternative to heat pumps.