i)To sprint, as fast as possible in a straight line towards your goal. Based on the scene in the popular movie, Forrest Gump where he just runs, and runs and runs.
ii) 21st century cockney rhyming slang. Going for a gump, means to go for a dump, or to excrete. Can also be heard as 'going for a Forrest'.
i) Look at that fucker gump. Jesus he's quick
ii) I'm just going upstairs for a quick gump.