Set the cutting height as high as possible. Go slow. Take only small bites. Don't try to mow a full-width swath at a time. Blow cuttings away such that they land on area you have already mowed. Rake cuttings up, if necesary. Once you get a first pass done, lower the deck to normal height and re-mow...

It won't hurt them. As suggested mow it high and maybe wait a while to mow it at the right height. This will let the clipping dry some. Go slow.

Pay close attention to "Don't try to mow a full-width swath at a time."
That is the absolute key to managing grass that has gotten out of control.
You should be able to judge right away how much your deck can efficiently handle by watching the discharge chute. If it spits out intermittent clumps of grass............you are taking too big a bite and moving too fast.
You might have to cut only only a 3rd or a 4th of your deck's actual width per each pass until you have chopped it up enough for a finishing cut later in the day.

Thanks for all the help. Got it done, took 1 hour with the Gravely, 1st gear (1 of 4), cut about 1/4 to 1/2 the width of the deck. More grass than I thought. So, it would have taken better part of a day with a string trimmer.

Gotta keep the grass mowed, mostly keep the ticks down. But found some bear poop down there too. Definitely too wild!!