these photos look like taken by a low resolution camera, but i think they aren't. so, i want to re-create that effect, does anyone know how to do that?
There are various plugins for Gimp that include old or poor photo effects to be obtained with a single filter action.
The probably most comprensive collection of filters come with the G'MIC plugins.
From the many effects avaliable the filter Degradations > Lomo or the filters Faded Analog or Vinatge from the Film Emulation set may come close to the effect you look after.
Here is an online version to test all filters: G'MIC Online