I'm typing this from distant memory, as it has been a long time since I've done any maintenance on my Gloria sprayers, so don't shoot me if my memory isn't up to scratch....
Unscrew the plastic cap that is at the top of the piston body. That will allow you to remove the plunger.
There is a big O-ring round the plunger seal at the bottom, it might have perished... what chemicals have you been using in the sprayer? I think these are Viton so should withstand most chemicals. I've only ever had to replace a couple over many many years of use.
The O-ring might have simply become dislodged. This is a sign of it running dry in the piston. You need to either re-locate or replace the O-ring and lube it up with silicone grease (example
here) and put plenty down the inside of the piston.. DO NOT use petroleum based grease, or anything marked "lube" from Graham Norton's bedside table
There is another possibility. At the very bottom of the piston there is a nut which you can unscrew. Do this over a towel on a work top, as bits might fall out maybe.
When you unscrew this, there is a brass poppet inside (ooh hello poppet, Graham Norton calling again!!). This little thing is the size of a couple of paracetomol tablets stacked on each other.
These poppets have a rubber internal part, it's a kind of non-return valve so you don't suck up the contents of your sprayer into the piston. This could have failed, in which case just replace it and it should sort the problem.
I must have confused you. I've confused myself
