A texture page is a single sheet (or multiple if it doesn't fit on one) that all your sprites get crammed into to fill up as much space as possible. Putting the animations in the IDE is making sprites as normal in the resource tree. GM takes those individual animations and fits them on a texture page to save space, then when you go to draw your animations, GM basically does "draw_part" of that big tex page to only display the frame of the animation it needs to. draw_sprite is really the same thing as draw_sprite_part except the draw_sprite_part gives you control over what part to display.
Is it possible to have a sprite sheet and automatically letting GM make the animations? Is that what you're talking about?
No, you can't just import a sprite sheet and have GM automatically draw the animation as it normally does. All GM knows is that this sprite sheet is another single sprite image. Besides, importing a sprite sheet wouldn't be very efficient if everything is in order and evenly spaced ect..because all that empty space isn't being cropped out or used at all. What you can do is use a program like Texturepacker for example, which creates full texture pages out of individual sprites (just like GM does), import that page in the Sprites folder, and manually draw_sprite_part each subimage at the right time to create the animation. This is essentially doing manually yourself what GM does for you when you simply create sprites in the resource tree.
I only use this method because Texturepacker is much better at packing textures than GM is able to do, thus saving me more memory. Not everyone needs to do this though, my project is very animation and resource intensive that I need all the savings I can get to create the experience I want to create.