Cover bread with cold water, and let soak for 15 minutes. Meanwhile, cover garlic with water in a small saucepan, and bring to a boil. Cook for 3 minutes; drain.
Pulse blanched almonds in a food processor until finely ground. Squeeze excess liquid from bread, and transfer bread to food processor. Add cooked garlic, vinegar, and 1 1/2 teaspoons salt. Puree mixture until smooth. With machine running, pour in oil in a slow, steady stream, alternating with cup cold water, blending until emulsified. Blend in remaining 2 1/4 cups cold water. Strain through a fine sieve until smooth, discarding solids. Season with salt. Refrigerate gazpacho until chilled, at least 1 hour (or up to 1 day). Season with salt and pepper.
Divide chilled gazpacho among 6 bowls or glasses. Drizzle with oil and top with grapes, and Marcona almonds if desired, just before serving.