Basically, you have 10 "levels" of Outsider which gives you +10 BAB and +7 base saves in all categories. And on top of that you have a 6 level adjustment, in the same way that the drow/half-fiend has a LA. You'll get the natural armor, the natural attacks, the special attacks (including spell-like abilities and summoning) and special qualities (including fast healing and various resistances/DR). Your class skills are all of the skills listed in the description of the Gray Slaad, and your skill points are (8+Int Mod) x 13. You should get 4 feats (levels 1, 3, 6, and 9). The necessary info that isn't in the stat block is in the 3.5 MM glossary under "Outsider".
If you need more help, go over to the rules forum and someone can possibly explain it better. You basically get to play the critter directly out of the MM, but you get a better ability score array, you get to pick your own feats, and you get to customize your skills. You also get whatever equipment a character of your ECL would have.
NOTE: I just noticed that you said 3.0. I used the stats out of 3.5 which are slightly different. In the 3.5 MM, Gray Slaadi are listed as 10 HD with a LA of +6. Savage Species (the 3.0 reference) lists the same ECL. I'd say to check with your DM and see how he wants to handle it, but using a monster as a PC isn't too complicated.
If you need more help, go over to the rules forum and someone can possibly explain it better. You basically get to play the critter directly out of the MM, but you get a better ability score array, you get to pick your own feats, and you get to customize your skills. You also get whatever equipment a character of your ECL would have.
NOTE: I just noticed that you said 3.0. I used the stats out of 3.5 which are slightly different. In the 3.5 MM, Gray Slaadi are listed as 10 HD with a LA of +6. Savage Species (the 3.0 reference) lists the same ECL. I'd say to check with your DM and see how he wants to handle it, but using a monster as a PC isn't too complicated.