Which gpu for Dell Optiplex Intel Core i5 3470

It will depend on the chassis, form factor of your OptiPlex.  The GPU can be a full height or low profile.

Then it will depend on the power supply in your OptiPlex.  The GPU will likely be a single slot, between 50w to 75w.

@Chino de Oro​  I mean can I still add gpu, I guess its ssf , here is the photo

It looks like an OptiPlex 7010.  You can add a single slot, low profile card with max of 50w or lower power consumption.  Some suggestion a low profile GeForce GT 1030.

You can learn more about system specifications and reference information from this technical guidebook https://i.dell.com/sites/doccontent/business/smb/merchandizing/en/Documents/OptiPlex_7010_Technical_Guidebook_1.pdf

a 75W gpu that has no 6 pin socket can do that even though the slot is for 50w "max".

for example gtx 1050 Ti Load Wattage:241 W
Recommended PSU Wattage:291 W