But does it have any practical application?

Being that the premise behind the studies are flawed there is no real practical application per se.  The studies are really focused on the effects of these foods on the bones based on the assumption that they cause acidity or alkalinity.  But again, diet has very little influence on the blood's pH.  Buffering of acidity by bone minerals is rare since this would take such an extreme acidosis to cause this.

Can eating too many alkaline foods or acidic foods be unhealthy?

There is really a lot to that question since again the diet has very little influence on blood pH.  But depending on the food the food can have adverse effects depending in part on amount.  For example, eating too many fruits can have a laxative effect causing problems.  They can also affect blood sugar creating problems for diabetics.  Too much milk can cause bone loss, heart disease,....  Too much meat can increase inflammation and possibly create an iron overload.  Too much raw cruciferous vegetables can suppress the thyroid.  Too many carrots when pregnant can cause uterine contractions.............

Then there are issues depending on other factors such as medications someone is on.  If on Coumadin most anything is an issue.  Grapefruit juice and milk also create problems with a lot of medications.  What a diabetic person can eat will be different than someone is healthy.  In fact there are a number of conditions that can influence what a person can and cannot eat.  For example those with Celiac need to avoid gluten products while a person with gout needs to avoid high purine foods.  Some people with rheumatoid arthritis are sensitive to nightshades and others can be sensitive to the salicylates in many foods.

None of this though has anything to do with whether or not the food is supposedly alkaline or acidic.  And if we really get down to it they are basing their premise on the effects of these foods on urinary pH.  Problem is that urinary pH does not reflect blood pH.  Neither does salivary pH.

How does any of this relate to GERD?...or does it?

Again, this has nothing directly to do with the blood pH, which is what they are claiming is being influenced by these foods.  The closest the blood pH is going to play a role here is that some of the carbonic acid in the blood is used to form stomach acid.

Acid reflux is most often from a lack of stomach acid.  But other factors are involved.  For example, a hiatal hernia makes acid reflux more likely due to stomach compression.  Age and the use of antacids and acid blockers can also increase the risk of acid reflux through stomach acid reduction.  This leads to less digestion and increased fermentation in the stomach.  The resultant gas formed by fermentation puts pressure on a valve called the lower esophageal sphincter (LES) that prevents food and acid from going up the esophagus.  This muscular structure though will eventually tire out from trying to hold back the pressure of the gas.  When it tires out the LES relaxes and the gas races up the esophagus with the traces of acid causing the "burn" of the reflux.  Certain foods such as high sugar foods can add more fermentable substances increasing the gas production thus increasing the risk of reflux.  And other foods such as mints, chocolate and some drinks like coffee all relax the LES also increasing the risk of reflux.  But again these have nothing directly to do with the pH of the blood, or in this case the urine they were testing the pH of.