The simplest way is to use the local storage to remember if the consent is accepted AKA "Set item" & "check item exists" actions.
For the pop up message, the simplest way is to have a new layer at the top. Give it a parallax 0,0 to make sure your message doesn't scroll or move around.
Then just have your Text object, button object and Tiled background in that layer. Perhaps at the bottom of the screen (below anchor settings assumes this).
Make sure you use the anchor behavior for all your objects in that layer if you're using "scale outer" for your fullscreen mode in project settings.
Have your tiled background and text object set to Left edge: Viewport left, Top edge: Viewport bottom, Right edge: Viewport right, Bottom edge: None.
Button object just be Left edge: Viewport right (assuming your accept button is on the right), Top edge: Viewport bottom.
Once player hit the button, remember consented and set layer to invisible:
+ Touch: On touched Button
-> LocalStorage: Set item "consent" to "true"
-> System: Set layer "Pop-up" Invisible
+ System: On start of layout
-> LocalStorage: Check item "consent" exists
+ LocalStorage: On item "consent" exists
-> System: Set layer "Pop-up" Invisible
we.tl/t-gpZs0Dhqdt [.c3p file]
Also make sure you follow the guidelines and modify the example file for your needs.
gdpr.eu/cookies