Thanks for your help, guys, but there are some things that do not seem right in afi's suggestion.
From a cursory examination of the jk2config.cfg file it seems that there is no "+jump" command but rather "+moveup" is used for jumping.
The command is also in between "" so perhaps the syntax there is wrong?
And mrwonko, I am not sure I understand your suggestion about learning entities and change them in all maps. There is only one map that would need a gravity change. Shouldn't it be a numerical value registered by the game when the map is loaded as suggested here?
"g_gravity "800" set the gravity level. (this is normally set by a property of the map loaded)"
Wikipedia gives a number of examples of what entities are:
"Specifying locations of various entities, such as player units, enemies, monster spawn points, ladders, coins, resource nodes, weapons, save points,[23] etc.;"
Is the gravity value an entity of the map?
Eventually I tried the command set g_gravity in the console but it does not work.
I get a message informing me that g_gravity is read only.
So it seems that the only solution is to change that value in the map. Do you know if there are any tools I could use to do that?
Thanks again.