There are inserts by Monroe and KYB available that will fit the Gamma struts assuming the strut casing is OK. It may be that your struts already have inserts but even if they don't, you need to remove the retaining nut/collar. To do that you either need to make a tool out of a pipe that will fit the collar slots, or use a cold chisel and large hammer and a lot of strength to remove the top nut/collar. Once you've managed to remove the retaining nut/collar and fitted a new insert, you can get a new nut from betaboyz to hold the insert in place
https://353652584127257704.weebly.com/store/p169/Front_Strut_Conversion_Collar.html