  Incoherent photon emission by ultrarelativistic electrons in the normal
incidence of a laser pulse on a foil is investigated by means of
three-dimensional numerical simulations in the range of intensities $2 \times
10^{21} \text{--} 2 \times 10^{25} \text{W} \, \text{cm}^{-2}$ and electron
densities $2 \times 10^{22} \text{--} 1 \times 10^{24} \text{cm}^{-3}$. We
focus on properties of the resulting synchrotron radiation, such as its overall
energy, directivity of the radiation pattern and slope of the energy spectrum.
Regimes of laser-foil interactions are studied in the framework of a simple
analytical model. The laser-plasma parameters for efficient gamma-ray
generation are found and revealed to be close to the parameters for
relativistic foil motion. It is shown that in the case of oblique incidence of
a $3 \text{PW}$, $10 \text{fs}$ laser pulse on a thin foil about $10^{8}
\text{photons}/0.1\% \text{bandwidth}$ are produced at the energy level of $1
\text{MeV}$ that significantly exceeds performance of the modern Compton
gamma-ray sources. Various applications of the gamma-ray bunches are discussed.
