  (abridged) Far-infrared (FIR) line emission provides key information about
the gas cooling and heating due to shocks and UV radiation associated with the
early stages of star formation. Gas cooling via FIR lines might, however,
depend on metallicity. We aim to quantify the FIR line emission and determine
the spatial distribution of the CO rotational temperature, ultraviolet (UV)
radiation field, and H2 number density toward the embedded cluster Gy 3-7 in
the CMa-l224 star-forming region, whose metallicity is expected to be
intermediate between that of the LMC and the Solar neighborhood. By comparing
the total luminosities of CO and [O I] toward Gy 3-7 with values found for low-
and high-mass protostars extending over a broad range of metallicities, we also
aim to identify the possible effects of metallicity on the FIR line cooling
within our Galaxy. We studied SOFIA/FIFI-LS spectra of Gy 3-7 covering several
FIR lines. The spatial extent of CO high-J (J>14) emission resembles that of
the elongated 160 um continuum emission detected with Herschel. The CO
transitions from J=14-13 to J=16-15 are detected throughout the cluster and
show a median rotational temperature of 170+/-30 K on Boltzmann diagrams.
Comparisons to other protostars observed with Herschel show a good agreement
with intermediate-mass sources in the inner Galaxy. Assuming an origin of the
[O I] and high-J CO emission in UV-irradiated C-shocks, we obtained pre-shock
H2 number densities of 10^4-5 cm-3 and UV radiation field strengths of 0.1-10
Habing fields. Far-IR line observations reveal ongoing star formation in Gy
3-7, dominated by intermediate-mass Class 0/I young stellar objects. The ratio
of molecular-to-atomic far-IR line emission shows a decreasing trend with
bolometric luminosities of the protostars. However, it does not indicate that
the low-metallicity has an impact on the line cooling in Gy 3-7.
