os/modules/graphical/sway/config/config.d/gaps.conf

# Set inner/outer gaps
gaps inner 10
gaps outer -5

# Additionally, you can issue commands with the following syntax. This is useful to bind keys to changing the gap size.
# gaps inner|outer current|all set|plus|minus <px>
# gaps inner all set 10
# gaps outer all plus 5

# Smart gaps (gaps used if only more than one container on the workspace)
smart_gaps on

# Smart borders (draw borders around container only if it is not the only container on this workspace)
# on|no_gaps (on=always activate and no_gaps=only activate if the gap size to the edge of the screen is 0)
smart_borders on

# Press $mod+Shift+g to enter the gap mode. Choose o or i for modifying outer/inner gaps. Press one of + / - (in-/decrement for current workspace) or 0 (remove gaps for current workspace). If you also press Shift with these keys, the change will be global for all workspaces.
set $mode_gaps Gaps: (o) outer, (i) inner
set $mode_gaps_outer Outer Gaps: +|-|0 (local), Shift + +|-|0 (global)
set $mode_gaps_inner Inner Gaps: +|-|0 (local), Shift + +|-|0 (global)
bindsym $mod+Shift+g mode "$mode_gaps"

mode "$mode_gaps" {
        bindsym o      mode "$mode_gaps_outer"
        bindsym i      mode "$mode_gaps_inner"
        bindsym Return mode "default"
        bindsym Escape mode "default"
}
mode "$mode_gaps_inner" {
        bindsym plus  gaps inner current plus 5
        bindsym minus gaps inner current minus 5
        bindsym 0     gaps inner current set 0

        bindsym Shift+plus  gaps inner all plus 5
        bindsym Shift+minus gaps inner all minus 5
        bindsym Shift+0     gaps inner all set 0

        bindsym Return mode "default"
        bindsym Escape mode "default"
}
mode "$mode_gaps_outer" {
        bindsym plus  gaps outer current plus 5
        bindsym minus gaps outer current minus 5
        bindsym 0     gaps outer current set 0

        bindsym Shift+plus  gaps outer all plus 5
        bindsym Shift+minus gaps outer all minus 5
        bindsym Shift+0     gaps outer all set 0

        bindsym Return mode "default"
        bindsym Escape mode "default"
}