  Observations indicate that most of the universal matter are invisible and the
gravitational constant $G(t)$ maybe depends on the time. A theory of the
variational $G$ (VG) is explored in this paper, with naturally producing the
useful dark components in universe. We utilize the observational data: lookback
time data, model-independent gamma ray bursts, growth function of matter linear
perturbations, type Ia supernovae data with systematic errors, CMB and BAO to
restrict the unified model (UM) of dark components in VG theory. Using the
best-fit values of parameters with the covariance matrix, constraints on the
variation of $G$ are $(\frac{G}{G_{0}})_{z=3.5}\simeq
1.0015^{+0.0071}_{-0.0075}$ and $(\frac{\dot{G}}{G})_{today}\simeq
-0.7252^{+2.3645}_{-2.3645}\times 10^{-13} yr^{-1}$, the small uncertainties
around constants. Limit on the equation of state of dark matter is
$w_{0dm}=0.0072^{+0.0170}_{-0.0170}$ with assuming $w_{0de}=-1$ in unified
model, and dark energy is $w_{0de}=-0.9986^{+0.0011}_{-0.0011}$ with assuming
$w_{0dm}=0$ at prior. Restriction on UM parameters are
$B_{s}=0.7442^{+0.0137+0.0262}_{-0.0132-0.0292}$ and
$\alpha=0.0002^{+0.0206+0.0441}_{-0.0209-0.0422}$ with $1\sigma$ and $2\sigma$
confidence level. In addition, the effect of a cosmic string fluid on unified
model in VG theory are investigated. In this case it is found that the
$\Lambda$CDM ($\Omega_{s}=0$, $\beta=0$ and $\alpha=0$) is included in this
VG-UM model at $1\sigma$ confidence level, and the larger errors are given:
$\Omega_{s}=-0.0106^{+0.0312+0.0582}_{-0.0305-0.0509}$ (dimensionless energy
density of cosmic string), $(\frac{G}{G_{0}})_{z=3.5}\simeq
1.0008^{+0.0620}_{-0.0584}$ and $(\frac{\dot{G}}{G})_{today}\simeq
-0.3496^{+26.3135}_{-26.3135}\times 10^{-13}yr^{-1}$.
