Great Weapon Master does not grant any weapon proficiencies
As others have pointed out, the Great Weapon Master feat does not grant you proficiency with heavy weapons (which is to say, weapons with the "Heavy" weapon property, as per PHB. p 147) as nothing in the feat's description says that it does, but rather, the feat already assumes you have proficiency in a heavy weapon.
If you do not have proficiency with a heavy weapon already, then you are not able to make use of that second bullet point. You are, however, still able to make use of the first bullet point, since, as is discussed in this Q&A, you don't actually need to use a heavy weapon to benefit from that first bullet point, so a wizard with a dagger would still be able to benefit.
However, if you find another way to gain proficiency with a heavy weapon, you would be able to make use of that second bullet point. The most obvious ways to gain such a proficiency would be to multiclass; multiclassing into a class that gives you proficiency with all simple weapons would grant you proficiency with the Greatclub, which is a simple heavy weapon, but not a great choice; better is to multiclass into a class that grants martial weapon proficiency, such as barbarian, fighter, paladin, ranger, etc.
The other most obvious way to gain weapon proficiencies is via the Weapon Master feat, which gives you proficiency in any four weapons of your choice. If you chose at least one weapon that was heavy, such as a greatsword, then you would be able to use the second bullet point of Great Weapon Master (although that's quite the investment, using up two of your wizard's ASIs to gain those two feats).
For other ways to gain weapon proficiencies, see these related questions: