Depending on which player you are, you use p#_name to change your name. open the console, and type:
p0_name "My Name"
By default, your player number is 0 (zero). However, if you are using splitscreen, the player on the left is p0 and the player on the right is p1. You can also add this into config.cfg so that gusanos has this settings every time you run it.
When changing your name, you can put in formatting controls. By pressing Control + K, you put in a little symbol of an arrow pointing downwards. You then put any of the following numbers after it: 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 Where 01 is black, and the bright colours are first, with them getting darker in the two digit numbers. To get a listing of all the different colours, bring up the speech box with F1, and replace the K in the following text with the control and k symbol:
k0101k0202k0303k0404k0505k0606k0707k0808k0909k1010k1111k1212k1313k1414k1515