Hi! What do you mean by game play mode?
Pressing G? or in-game? something else?
Edit: gotta go.
In case “gameplay mode” means clicking play (Play in selected viewport, Play in new editor window etc), that’s a game so it needs to be scripted. You need to tell the game code what you want to do. Blueprints or C++. Something like character with camera and changing character speed. In case you’re using a default template from Epic, like Third person or First person template, open up the character and find Character movement in components tab, and edit Max walk speed (you can search for it from the top of the panel).
In other case gameplay mode means something related to editor like after ejecting during play, you can increase / decrease speed of fly over controls with scroll wheel (the same what works in viewport).
Good luck