People have a lot of interesting ideas about what will raise or lower their blood glucose levels. The other day I was dealing with a particularly trying individual whose blood glucose levels were so high his glucometer (blood glucose measuring device) would only read "HI," meaning that his readings were over 600. This is the point where I as a medical professional plead with the person to go to the hospital as he is risking dehydration and death, but he was agreeable to increasing his insulin dose slowly day by day to get his readings under control.* He also said he was going to start "eating pickles and garlic to bring my sugar down, just like my daddy said."
*I'm sure there are some people who are going to read that line and start accusing me of just wanting to give people medicine instead of going the "natural route." First of all, in a situation like this one--having a person just change their diet to lower their blood glucose levels is the equivalent of standing over someone who is having a heart attack and saying "Dude, you really need to eat more vegetables" instead of dialing 911 and/or performing CPR. Second of all, people have to actually FOLLOW a reduced carb meal plan to lower their blood glucose and this guy was clear he knew all about it and wasn't going to do it anyway.
Anyway, I've heard the pickles and/or garlic myth before, and I think I have a pretty good handle on where that might have come from. As far as the pickles go--if you have someone who has been eating a lot of salty tasting snacks like potato chips or pretzels, and they substitutes pickles, they have just went from a high carbohydrate consumption to a lower carbohydrate consumption and the removal of the all of that carbohydrate probably is what helped. Nothing magic about the pickles. Now, interestingly enough, there is some limited evidence that regular consumption of vinegar (which is in pickles!) might result in lower post meal blood glucose levels (Link); but we certainly don't know if a steady dose of pickles after your dose of potato chips will continually control your blood glucose.
Garlic is a different story in that there seems to be no evidence that eating garlic or using it as a supplement will actually help lower your blood glucose. (Link) Now, once again, some people will just eat garlic as is, and other people are probably going to use it as a flavoring on their meats or vegetables. If using garlic as a flavoring helps you eat more vegetables and less carbohydrate, by all means add the garlic, but don't expect it to magically bring your blood sugar down.
Take home message--substituting pickles and garlic on vegetables for high carb snacks and high starch meals could help keep your blood sugar from going to high, but it won't bring it down if it's already high.