Here's an idea to remove the glue.
Get some liquid frisket at an art supply store (it's not expensive; it's mostly used to keep white spaces on water-color paintings and peels off easily). Paint that around the glue to protect the surrounding painting. Then apply water to the glue - dollars to donuts it's water soluble.
Take your time, do it in stages and be gentle, cross your fingers and you may be able to get all the glue off the painting without harming it.
Great find!
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as an after-thought, a bit of glue on the work of an artist who uses a lot of glue, isn't a huge deal, so you don't have to make it pristine