Hi there, let's look at widths first - the Gecko is the original one, 120L = 77cm? and you are considering the Blast 100 (62cm)/115 (66cm), or the newer 2022 models 100 (unchanged 62cm)/117 (67cm). Gecko 100 is 66cm wide.
So it depends a bit on your skill level/weight and water conditions somewhat. All three Blast/Gecko versions will work well with the sail sizes, the 100 Blast as the most narrow probably best with 5.2/4.7 sails, but then not as good with a 6.4 compared to the Blast 115/117 and Gecko 100. If you are still progressing, then I would go for a slightly bigger board in terms of volume than the Blast 100 - up to you then if you want a more performance oriented model - go for the Blast 115/117. If you want a board which feels fairly similar in terms of control, ease of use, to your existing Gecko 120 - then go for the Gecko 100? This will work well with all three sail sizes and is a very forgiving shape, but also more performance due to smaller size than your 120?