it's hard to tell how your tiles look with all the default tiles "clogging" the screen
but my first notion is that the tiles you made are very monotonous, or in other words they are all very similar. try adding some variation to your tiles, like two or three copies of each tile, to break everything up and have everything look more natural.
for example, look at this stock image of a brick wall:
it has varied shapes and colors; some bricks are cracked and faded; some bricks are more inset than other bricks. variation is what makes it look realistic.
take a look at the default tiles gamemaker, provided, even. there are cracks in the walls, crack tiles on the floor. these are all tiles, too, but they're not the default, generic tiles. instead, these tiles are used to give personality to the setting.
another lesson you can learn from gamemaker's default tiles is their use of hue. the tiles are very saturated (more grey), just like yours. but the default tiles also add a green-ish hue to the wall, which gives it character. grey walls are boring, because they tell us nothing about the building or environment, except maybe that it's boring, too. green walls tell me that the environment is in nature, or that the environment is plagued, or that the environment is on the sea in some way.
the fact is: color speaks volume. imagine if i painted a dog in a bright red and then in a dull brown. a bright red dog would be angry, menacing. a dull brown dog would be subservient and loyal. the same should go for your walls. what are they? where are they? why are they there? this should influence the design of your walls, and also, what little bits like cracks you add to them, too.
both color and detail in mind, i made a quick, rough wall, which should serve to illustrate my points. it's not tile-based, because i'm short on time, but it should show what i meant well enough.
i wish i had a bit more time to make a better edit, but i'm not a great tile-r myself, so id need a while. nevertheless, think about variation and hue to your tiles to give them more interest in the viewer's eye.