I don't think Nexium causes gastritis. If anything, it probably isn't helping you if you are still having problems. I'm not sure about these candida clenses - I don't think there is much research support for it.
The majority of GERD symptoms are caused by a low esophageal sphincter pressure, which allows gastric acid, and sometimes bile (from you gall bladder) to reflux into the esophagus. Proton pump inhibitors have generally been found to be safe, and are the current standard therapy for GERD. Chances are that eating "alkaline" or "acidic" won't help too much because the problem is with the sphincter, and not necessarily with the amount of acid in the stomach - and every food you eat will stimulate the production of stomach acid. Certain foods can lower the sphincter pressure, which you want to avoid (chocolate, caffeine, etc.), but bland diets (those low in acidic foods) haven't typically been found to be really helpful.
One thing is to be sure - it is not good to have chronic heartburn, as this signals esophageal irritation. Chronic esophageal irritation can lead to serious consequences, and must be avoided. So, if you are still having troubles, I would talk with your GI doc and see what the other options are.