I didnt see any guides on this so here you go.

Open the console by pressing ` (if this doesnt work make sure to check if youve set: Setings -> Game -> Enable Console -> Yes).

Type the following commands..

/g_gametype gun , this wil change the gamemode to gun game.

OPTIONAL , if you want to change the match time you will need to use the command: /scr_gun_timelimit X (0 will make it so there is infinite time).

/map_restart , this will force the map to restart. When it restarts you will be playing gun game!

Enjoy yourself cμnt or whine in the comments that this didnt work for some reason and ill help you out bb <3.