While not exactly the same, “equivalent gas use” can be calculated from the heat output divided by 90%. If the home is being heated to the same temperature, then this is how much gas would have been consumed, assuming that a gas boiler system is 90% efficient - this has been described as “generous”, as many systems won’t be anywhere near this good, but it’s a reasonable number for comparisons.
I have an experimental dashboard that shows running costs of heat pumps (for current UK prices).
See this post for the analysis of running costs I did a little while back.