My own 2p worth:
Dont do it OP. I can confirm from personal tests carried out on my own filter that they contain small amounts of white asbestos in the lower layer (the HEPA particulate filter). In theory it is safe to wear as the particles are trapped within the papers material and the two layers of cotton and layer of charcoal make it nearly impossible for a fibre to get through. In reality you do not know how much that filter has degraded over time, and even if it looks fine it may not be, and there i always a chance of the fibres getting through even if it is in perfect condition. I wore mine for a few times before I found out and although my lung scans came back clear I am dreading what may yet some to pass.
Even if you get it emptied out (and properly cleaned, just removing the contents is not enough to ensure safety) you still have the problem of the paint. You see that nice, shiny, paint on the filter? That is lead paint. Enjoy.
TL;DR dont do it. Get a hose and thoroughly detalc it and use that instead. Have the hose feed into a bag or pouch or pocket or down your top. Just do not use a GP-5 filter. If you do want a useable filter then purchase an FP-5 filter, its a modern universal thread filter that is safe to use.