“Raster Gothic Condensed is the font used throughout the Pebble system, largely because it is optimized for monochromatic displays. Pebble selected this font because it allows a relatively large number of characters to be displayed on a single line, also because the font has an excellent readability vs. size ratio.
The font family was created by designer and typographer Mark Simonson as a series of outline fonts that retain the aliased look of a bitmap font. The advantages for the Pebble app developers are notable: clarity and crispness of watch display with a smaller, optimized memory footprint.” — Pebble Developer Guide
The other typeface available in the Pebble OS is Bitham, a bitmap version of Gotham.