The "that advantage" the person was mentioning was most likely

Sounds obvious. When you have advantage, there is a good chance for you to hit. If you use GWM/SS, you lose that advantage.

Meaning that it decreases your "good chance to hit", back to the usual chances you usually would have.

#No, you don't lose your mechanical advantage So you can still use things like Sneak Attack or any other thing that depends on the condition "having advantage".

Actually, even if the probability of hitting the target is about the same as having no advantage or penalty at all, the "right way" to resolve this is still

1. Roll 2d20
2. Take the greater value
3. Add the appropriate bonus
4. Reduce the 5 penalty
5. Check if you hit

Meaning you still roll the 2 dice from advantage.

###Related

It is similar to how the Lucky feat turns Disadvantage into "Super Advantage", but the "Disadvantage" status is still there.

The "that advantage" the person was mentioning was most likely

Sounds obvious. When you have advantage, there is a good chance for you to hit. If you use GWM/SS, you lose that advantage.

Meaning that it decreases your "good chance to hit", back to the usual chances you usually would have.

No, you don't lose your mechanical advantage

So you can still use things like Sneak Attack or any other thing that depends on the condition "having advantage".

Actually, even if the probability of hitting the target is about the same as having no advantage or penalty at all, the "right way" to resolve this is still

1. Roll 2d20
2. Take the greater value
3. Add the appropriate bonus
4. Reduce the 5 penalty
5. Check if you hit

Meaning you still roll the 2 dice from advantage.

Related

It is similar to how the Lucky feat turns Disadvantage into "Super Advantage", but the "Disadvantage" status is still there.