I am no expert but yes, a gold one will warm things up, but what about a shoot through umbrella ?
A couple of small gels would enable you to warm up the flash a little and might be more versatile in the long run.
As you say you will be shooting in afternoon light there may be some warm light anyway and a gold brolly might just push things too far ?
You may get a reply from one of the experts like Garry Edwards if you were to get this thread moved to the lighting section - I think you can ask the mods to do this for you !