Yes, you could get another batten made up for your 5.2 Gaastra (which model if you know?) but there are sails in the 5.6 m2 range that might cover both your 6.0 and 5.0 needs.
Sailworks 5.6 m2 Hucker is nearly as powerful as most 6.0's, and it can be detuned (LOT'S of downhaul here) to a bit smaller than 5.6 (maybe as far down as 5.2 m2).
The Huckers are easy to rig, and if you have a couple of different masts, you can extend the range down a bit more by changing to a softer mast.
RDM masts seem to "power up" the Huckers on the low wind end, and std. dia. masts give better high wind stability, with good low wind power.
Huckers are not for everyone, as they have a bit of an old style "more power in the top" feel, but for your FL conditions, the 5.6 Hucker could work really well.
Hope this helps,