I believe that it's a rare
English word.*
Perhaps as rare as those fighting gals themselves were, certainly not as common as in Russell Whitfield novels; I suppose part of their appeal was in that gladiatorial shows with their participations were a special treat.
*Unexpectedly, for me at least,
OED dates it to 1802!
Well, that's sent me off on a typical eul library hunt - the OED citation is from one of William Gifford's fascinating footnotes to his very lively translation of Juvenal's Satires, specifically 1 vv.34-5. Unfortunately I can't track down the Latin original, but Gifford's translation reads:
'When Maevia, all the woman laid aside,
enters the lists, and, to the middle bare,
Hurls at the Tuscan boar the quivering spear...'
Juvenal at this point is listing all sorts of deplorable antics of the wealthy Romans of his day. Gifford begins his footnote:
'Under Domitian such instances were common, for he not only exhibited combats of men with wild beasts, but of women also; and the noblest of both sexes were sometimes engaged in them!'
He then goes on to correct Dryden, who thought Juvenal was referring to an actual hunt rather than a woman vs boar contest in the arena, then concludes:
'Of Maevia I can find no account: there is indeed a strumpet so-called in Martial, but she was poor: her profligacy, however, may have tempted Juvenal to transfer her name to this noble
gladiatrix.'
Although
gladiatrix isn't found in Classical Latin (to judge from Lewis & Short, I haven't got access to the online Oxford Latin Dictionary), it's been used pretty frequently in most major European languages, this search on Google Books turns up a huge number (257!) of links:
https://archive.org/search.php?query=gladiatrix&sin=TXT
(though quite a few are to species of moth or fly that have gladiatrix as their specific name)