There are a few ways you can tweak the server to offer certain gameplay changes.  If you start a server using one or more of these modifications, please follow the server naming code described in the instructions thread.

Time limit (command line):

?timeLimit=X
The default time limit is 0, meaning that the match will not end until one team has 160 points.  In 1v1 matches it can take a very long time to reach 160 points, so you can set a time limit in minutes.  A timer will show at the top of the screen, and the match will end when time runs out.

Example:
start SGBGame.exe server TDM-Amarna.umap?timelimit=15?game=SGBGame.SGBTDMGameInfo -SERVERNAME=xx4xxxx -MAPNAME=Temple -DEDICATED=true -CONNECTIONSTRING=x.x.x.x:777 -PORT=7777

Game Speed (command line):

?GameSpeed=X
The gamespeed will affect all aspects of the game, including player movement, firing speed, damage effect duration, etc.  The default game speed is 1, and you can change it to a higher or lower number.  Decimals can also be used. (ie. 1.25, 1.8, etc)

Example:
start SGBGame.exe server TDM-Amarna.umap?timelimit=15?game=SGBGame.SGBTDMGameInfo?GameSpeed=2 -SERVERNAME=xx1xxxx -MAPNAME=Temple -DEDICATED=true -CONNECTIONSTRING=x.x.x.x:777 -PORT=7777

Gravity (configuration file):

DefaultGravityZ=-X.X
To change the gravity, you will need to edit a configuration file.  Therefore, the change will affect all the servers that you start.  If you want to run servers with and without gravity tweaks at the same time, you can make a second copy of the sgr folder.
To make sure you don't forget about the change, i suggest naming the new folder something like SGRgravity or SGRtweak, and you can use the new copy for all your tweaked servers, while keeping your original copy unmodified.

The configuration file that you need to edit is named SGBGame.ini and it's located in \sgr\SGBGame\Config

Look for the [Engine.WorldInfo] section, and edit the following line:
DefaultGravityZ=-750.0

Note that the value is negative, and the closer you get to zero, the lower the gravity will be.  I've found that -150 is a good setting for low gravity matches when combined with increased game speed, but you could try lower values like -700 or -650 if you just want to be able to jump a little higher.

Values of 0 or higher will be weightless and therefore aren't very useful since you'll never fall back to the ground after jumping.