  As long-lived quasi-solitons from the fragmentation of a scalar condensate,
oscillons may dominate the preheating era after inflation. During this period,
stochastic gravitational waves can also be generated. We quantify the
gravitational wave production in this period with simulations accounting for
full general relativity to capture all possible non-perturbative effects. We
compute the gravitational wave spectra across a range of choices of the
oscillon preheating models and compare our results to a conventional
perturbative approach on an FLRW background. We clarify the gauge ambiguities
in computing induced gravitational waves from scenarios where dense
non-perturbative objects such as oscillons are being formed. In particular, we
find that the synchronous gauge tends to contain large artificial enhancements
in the gravitational wave spectrum due to gauge modes if gravity plays an
important role in the formation of the oscillons, while other gauge choices,
such as the radiation gauge or a suitably chosen "1+log" gauge, can efficiently
reduce the contributions of gauge modes. The full general relativistic
simulations indicate that gravitational wave spectra obtained from the
perturbative approach on the FLRW background are fairly accurate, except when
oscillon formation induces strong gravitational effects, for which case there
can be an order unity enhancement.
