Unused area on layout
Type "Change", then select "Orphans", then "on". If you click now on your polygon pour, it will fill the gaps. Be aware, they are not connected to ground, just blank copper. Another option is to reduce the "Isolate" value; that will allow your pour to move closer to the traces/holes. Check your manufacturing limits if you try that. Most boards these days are two sided, so it's usually possible just to put a via through. (There's very little price difference for hobbyists to go two sided over single sided, worth thinking about.)

Based on your layout, there is a few things you could do without changing the board size. If you move the component (resistor?) in the middle above the IC left and down a little, you could move the trace which goes underneath it down a little as well. That way, the GND pour might be able to fill in sufficiently.