James, did you use wordreference? It is good for many things but has errors and misdirects too. For < trempage > in the French-Anglais listing I see that it’s description is “nf” - however it’s example sentence is < Le trempage dans ce liquide ...  >, and in the Anglais-French it is correctly noted as “nm”. There are other occasions I have found “errors”, some of these may be typographical or editing errors, but some can arise because of differences between French (France) and French (Canada). 

Apart from Larousse and Robert, there is also the Académie Française site which is (surprisingly) easy to use :

https://www.dictionnaire-academie.fr/article/A8T1153

( Interesting to note from Laura Lawless is that all French nouns ending -isme are masculine. Unfortunately that relatively small group of nouns is the only completely predictable group for grammatical gender based on spelling. )