The external Gravity Load feature, prevents the user from selecting planes of suitable features to define the direction. when you select a feature, it seems to take a extremely long time to register the feature, and then when pressing OK, it simply asks to select a feature for reference again.
This seems to be a fundamental bug and is happening on two different machines and in SW2019 SP1 and SW2019 SP2.
Need this sorted out quickly, as it makes the FEA outputs useless until resolved, as there is no other way to apply acceleration fields.
I have tried it on my machine with 2019 SP2, but can't reproduce the issue. It seems to be model/workflow specific.
Another thing is that you should contact your reseller. They will check the issue and if it is a bug they will report it to SW.