@sensibleyid: we dont pasken like rebbi that tchelis is me’acev.
The askenazi rishonim pasken like Rava that all minim are d’raysa, but the Sphardi rishonim pasken like R Nachman and Tana Dbei R Yshmael that only Wool and Linen are D’oraysa.
(FYI, the Gaon wore Linen – not cotton, big difference! because his other undergarments were linen and he didnt want shaatnez – but linen is also d’oraysa for sure;
but Rabenu Tam held wearing linen is Assur mdrabanan because might wear tchelis (wool) on it on a nighttime garment – which wont be doche the lav of shaatnez)