gateway omg old or is Acer made, now. made 2005, 14 years old.
omg2, relic Pentium D PC, wow. old.
there are 2 ways, mostly, (besides rear clam shells)
or the latch.
we are USA are you outside USA the USA GW lists no 835.
usa has 830. and 840. we pitch these the trash, no w10 support.
this looks like slider case to me. the rear side panels do not just pop off 1 by 1, no.
the clue If you look the top and sides are 1 part.
so that top rear screw release it, or the who front comes off pops off and screws seen there release the wrapper.
that is 1 odd duck case. it retire it in a heart beat.
some odd cases tilt the pc from top to rear look bottom front/s
see levers there in plastic and when tripped the whole front tilts off.