Gabelvampir: Yes you can, but the GOG config already has the native resolution of your monitor as the fullscreen resolution configured.
But what's also important is the scaler used to get to game to that resolution. DOSBox can't influence what resolution a game renders at, it can only take the output and scale it up in different ways. The DOSBox has an article about the supported scalers (by coincidence they use a picture of the Doom Guy to illustrate them)
http://www.dosbox.com/wiki/Scaler. If you want to try some of the out set the with the GOG DOSBox configurator or edit dosbox_doom.conf in your Doom directory.
MaorOvadia794: LOL
i think its simple with GZDOOM with dosbox its to complicated
one more thing when i download gzdoom i extract the files and copy and paste it in ultimate doom folder ?
and what to change in GZDOOM settings for best picture when i enter to the game ?
It's a matter of taste mostly (some people like the filtered look some don't for example), there are a few presets in the gzdoom options IIRC.