I can perfectly understand him, a lot of my fathers generation in the villages around Chippy spoke similar to Gerald, especially in the farming communities. There can be a tendency to not finish words or sentences clearly, mumbling a bit if you like, if you‘ve grown up with it I guess your ear is attuned to it, however I get why ‘outsiders’ would struggle to understand Gerald. I agree with
@bashamwonderland that it’s been deliberately edited to make him come across as the local Village idiot.
The West Oxon/Gloucestershire border rural accent is less strong now for sure, and sadly in time will die out.