I used my digital camera to take photo, but I forget to switch off the timestamp function. Now every of my photos have the timestamp in it....How or Can I remove it using GIMP/Photoshop?
You cannot "remove" them from the top of the picture, as now they are an integral part of the picture itself. they are existing pixels of color.
Use the clone tools , you can patch surrounding stuff in, or various fix/patch tools, I prefer the clone tool because of manual arting in something that patches the area. You could potentially blur the area some , and mix it in some, or use burring/softening to hide some of the cloning.
Crop it out, they are usually low and to the side, many pictures can be cropped, find the ones that could be re-framed and get rid of some of them that way.
Depending on the picture , some of what defines the text overlay is the color, feather select the area , and trim back the color that the text is in, to reduce its appearance some. Ex: Red texts select the area (feathered) and pull the red down some in levels or color controls. it wont remove it, but it can dampen it more.
Think different, instead of removing it, replace it with a logo , funny sayings, or text that describes the picture. 20 years from now those dates can come in handy :-) Any text you toss on there now to describe the picture, the people in it, and stuff like that comes in useful when your memory starts to fail :-)