One of the unsolved problems in cosmic ray (CR) physics is the determination
of sources and acceleration mechanism(s) of Galactic ($E\lesssim 10^{18}$) eV
and extragalactic ($E\gtrsim 10^{17}$ eV) CR . Ultra high energy CR (UHECR,
$E>10^{18}$ eV) are believed to be of extragalactic origin, but some
contribution from transient Galactic sources is possible.
So, magnetar wind nebulae (MWNe), created by new-born millisecond magnetars,
and magnetar giant flares are PeVatron candidates and even potential sources
of UHECR. Promising signature of effective acceleration processes in magnetars'
neighbourhoods should be a nonthermal high-energy and very high-energy
$\gamma$-ray emission according to pp collisions with a subsequent pion decay
(hadronic scenario) and inverse Compton scattering of low energy background
photons by ultrarelativistic electrons and positrons (leptonic scenario).

In this work we explain a HE and VHE $\gamma$-ray emission from
the vicinity of the magnetar SGR 1900+14 -- potential Galactic
source of $E>10^{20}$ eV triplet -- by hadronic and leptonic emission of
cosmic rays accelerated in a magnetar-related Supernova remnant (SNR)
and/or MWN. To this end we carried out a simulation of the
observed HE and VHE $\gamma$-ray emission, spatially coincident
with the magnetar SGR 1900+14.
The extended {\it Fermi}-LAT source 4FGL J1908.6+0915e, the extended H.E.S.S.
source candidate HOTS J1907+091 and the point-like HAWC TeV source 3HWC
J1907+085 were considered. We show that the observed $\gamma$-ray emission
from the magnetar SGR 1900+14 outskirts may be explained by a (still
undetected) magnetar-connected super-luminous Supernova (Hypernova) remnant
and/or a MWN created by new-born millisecond magnetar with a large initial
rotational energy $E_{rot}\sim 10^{52} erg$.