First, it is not that easy. The average GP needs to produce close to 1 million a year to take home 330k. Most GP's don't produce 1 million a year. The ADA puts the average GP practice producing around 600k a year.
Second, it costs about 30 grand to get ready to do implants. Most implant courses will run you 8-10 grand. You will want to do a couple of those before you place your patients health and your license on the line. Next, it is around 10 grand to get your office set up to do implants. Finally, your overhead is fairly high to do implants (meaning, you probably put more money in your pocket doing 4 crowns in two hours than doing one implant in 4 hours), and that can go higher if you screw up along the way (ie, need a custom abutment; which happens a lot).
That being said, if you are competent in placing implants, you can make a lot of money. BUT, it is a lot harder to make 330k a year as a GP than it is as a specialist.