We present astrometric monitoring of the young exoplanet HD 95086 b obtained with the Gemini Planet Imager between 2013 and 2016 .
A small but significant position angle change is detected at constant separation ; the orbital motion is confirmed with literature measurements .
Efficient Monte Carlo techniques place preliminary constraints on the orbital parameters of HD 95086 b .
With 68 ~ { } \% confidence , a semimajor axis of 61.7 ^ { +20.7 } _ { -8.4 } au and an inclination of 153 \fdg 0 ^ { +9.7 } _ { -13.5 } are favored , with eccentricity less than 0.21 .
Under the assumption of a co-planar planet-disk system , the periastron of HD 95086 b is beyond 51 ~ { } au with 68 ~ { } \% confidence .
Therefore HD 95086 b can not carve the entire gap inferred from the measured infrared excess in the SED of HD 95086 .
We use our sensitivity to additional planets to discuss specific scenarios presented in the literature to explain the geometry of the debris belts .
We suggest that either two planets on moderately eccentric orbits or three to four planets with inhomogeneous masses and orbital properties are possible .
The sensitivity to additional planetary companions within the observations presented in this study can be used to help further constrain future dynamical simulations of the planet-disk system .