Means that it reverted back to default gap limit or just need time to display as "within the gap limit",
as mentioned above, those "marked red" are addresses beyond the wallet's gap limit (gap limit is saved in the wallet file itself);
However, it actually means your current gap_limit is only up to the last non-red-highlighted address.
If you'll use the wallet in that state, it will only generate 20 more addresses (instead of another 1000) after the 1000th is used.

Restart Electrum first, then check if those addresses are still red, if still red, there may be something preventing the setting.
But I bet it just needs a restart/time.