Does presence of an elevated anion gap mean that there is a metabolic acidosis?

Generally yes but not always.  Aside from the organic metabolic acidoses, an elevated anion gap can also be caused by hyperalbuminemia, hyperphosphatemia or presence of an anionic paraprotein (IgA monoclonal immunoglobulin) but given that these are fairly rare occurrences, once ruled out,  metabolic acidosis is the presumed etiology of an elevated anion gap.


Source

Emmett, M.  "Serum Anion Gap in Conditions Other than Metabolic Acidosis"  Up to Date.  Nov 2012.