Lemons are not alkaline. They are alkalyzing. See?
Its their acidic content that causes this effect, in fact.
Moving on.
Potassium citrate is fine, but be aware of dose.
About the GBL. GHB does not cause acidosis at any appreciable rate, although it has a VERY slight impact. So an alkalyzing diet is certainly best.
GBL, however, requires the use of an OH ion to convert to GHB, and it gets this from the blood. So you can get acidic very quickly, depending on your usage.
In fact, the severe symptoms described with constant GBL use, i.e. horrific withdrawals, sickness, confusion, lethargy...are more likely a result of metabolic acidosis.
Baking soda can work, but there is the side effect of sodium loading. Again, potassium citrate is key. This is also true because GBL leeches potassium from the body very quickly also. A related effect, actually.
I am able to stave off acidosis by supplementing my diet with wheatgrass, grapefruits, and watermelon. I have used a great deal of GBL. I have gone acidic from it, once. And once is enough, believe me. You will KNOW when there is a major dip in pH, trust me. You will feel shaky, lethargic, weak, paranoid, anxious. BAD.
If you get in this state, fire some greens and lemons and, you got it, potassium citrate.
About the baking soda. Never take it on a full stomach, ever. Also, be certain it is completely dissolved in water. No white, all clear before drinking. Drink it slowly. Use no more than one measured teaspoon at a time, and no more than once every two hours, for no longer than three days.
Like Animal said, it is not the first choice. But it works in a pinch, I can attest first hand.
BTW, being too alkaline can be deadly as well. Know your ranges and doses. They vary from person to person.