The goalie reacts according to his attributes. If you are controlling Gretz or Roenick, it's up to you to deke him out, using their speed, agility, stick handling, shooting, etc attributes. There isn't a probability you will score. You have to control the game and beat the goalie.
The players have base attributes. Gretzky has 4 speed and 6 agility (6 is the maximum for all the attributes except weight and fighting). Roenick has 5 speed, 5 agility, as well as 5 shooting power and 5 shooting accuracy. Other attributes are offensive awareness and defensive awareness (inputs to the player AI, which I've heard represent basically how far the player can see), endurance (for line changes), passing accuracy..
When the game starts, the code reads in those values, multiplies them by 5 to give a value from 0-30, and then applies a little bit of randomness to each one (from -3 to +2). These values are used by the game to control how fast the player moves, how hard their shots are, etc.
Depending on if the player is on the home team or away team, some player attributes get additional bonuses/penalties, depending on the home/away rating of the team. I don't remember which attributes are affected.
Teams also have powerplay and penalty killing ratings, which affect some player attributes during those game scenarios.
Finally, it is suspected, and vaguely confirmed, that there are some other hidden effects, which people on here often refer to as "momentum". Supposedly as the game goes on, it's more likely for goals to be scored. I don't know if this means goalie attributes are reduced, or player attributes are increased, or what.