The oil guage is screwed into a threaded receiver on the tank-top which is an integral welded part of the tank and it is probably a 1 1/2" coarse pipe thread, and maybe sealed with thread sealant. So, a 14" or 18" pipe wrench should easily unscrew it without doing any damage. It has a long 36" extender-arm w/ a float that's all attached to the guage fitting. It should all come out as one long piece.
You can do it! ...