  We present deep rest-frame UV spectroscopic observations using the Gran
Telescopio Canarias of six gravitationally lensed Lya emitters (LAEs) at
$2.36<z<2.82$ selected from the BELLS GALLERY survey. By taking the
magnifications into account, we show that LAEs can be as luminous as L(Lya) =
30x10$^{42}$ erg s-1 and M(UV) = -23 (AB) without invoking an AGN component, in
contrast with previous findings. We measure Lya rest-frame equivalent widths,
EW(Lya), ranging from 16\AA to 50\AA and Lya escape fractions, fesc(Lya), from
10% to 40%. Large EW(Lya) and fesc(Lya) are found predominantly in LAEs showing
weak low-ionization ISM absorption (EW < 1\AA) and narrow Lya profiles (< 300
km s-1 FWHM) with their peak close (< 80 km s-1) to their systemic redshifts,
suggestive of less scatter from low HI column densities that favours the escape
of Lya photons. We infer stellar metallicities of Z/Zsun ~ 0.2 in almost all
LAEs by comparing the P-Cygni profiles of the wind lines NV1240\AA and
CIV1549\AA with those from stellar synthesis models. We also find a trend
between M(UV) and the velocity offset of ISM absorption lines, such as the most
luminous LAEs experience stronger outflows. The most luminous LAEs show star
formation rates up to 180 Msun yr-1, yet they appear relatively blue
($\beta$(UV) ~ -1.8 to -2.0) showing evidence of little dust attenuation
(E(B-V) = 0.10-0.14). These luminous LAEs may be particular cases of young
starburst galaxies that have had no time to form large amounts of dust. If so,
they are ideal laboratories to study the early phase of massive star formation,
stellar and dust mass growth, and chemical enrichment histories of starburst
galaxies at high-z.
