could you provide with complete model number please? you seem to be missing a few digits on end of model number
example Gateway Desktop PC SX2185-
UR21 <---- this will help identify what motherboard and CPU you have, thanks
I did find your User manual:
https://www.manualslib.com/manual/1053976/Gateway-Sx2885.html#product-SX2185
Looking on ebay at a
Gateway SX2185 AMD Desktop Motherboard replacement for SX2185-UR24, SX2185-UR21, SX2185-EB30, SX2185-MT30, I clearly see a PCIE Slot for a Video card, again need to confirm your complete model number but tentatively I would say yes you can expand to a new video card.
the biggest problem I see before answering your question is that out of all the models I looked up, all the power supplies provided where horrible under powered. the SX2185 is powered by an Acer Delta 220 watts power supply.
and this brings out a possible problem, as if we calculate the power consumption for your CPU/Motherboard/RAM
OuterVision PSU Calculator part list
Motherboard: Desktop
CPU: 1 x AMD A6-5200
CPU Speed: 3600MHz
CPU Vcore: 1.3V
CPU Utilization: 90%
Memory: 1 x 4GB DDR3 Module
Storage: 1 x SATA 5.4K RPM
Optical Drive: 1 x DVD-RW/DVD+RW Drive
Keyboard: 1 x Standard Keyboard
Mouse: 1 x Standard Mouse
Computer Utilization: 8 hours per day
Load Wattage: 170W
Recommended Wattage: 220W <---- the power of your PSU
Generated by OuterVision PSU Calculator 2017-01-21 01:44:29
and this shows clearly you have not enough power in your power supply to run another video card safely
so Yes you can get another video card, but you will ALSO need to change the power supply.