Fix broken conditional in gpg.sh

[grdryn]

Using if [ works differently than if [[ (the former single square
bracket is a file, the double one is a builtin function (I think)).

When using if [ with -eq, it expects numbers, but == can deal
with strings, which is what we want here.

Some examples:

$ if [ hello -eq hello ]; then echo yes; fi
bash: [: hello: integer expression expected

$ if [ hello == hello ]; then echo yes; fi
yes

$ if [ 1 -eq 1 ]; then echo yes; fi
yes

$ if [[ hello -eq hello ]]; then echo yes; fi
yes

$ if [[ 1 -eq 1 ]]; then echo yes; fi
yes

Note: this should at least fix the initial problem, I'm not sure if there are others further on. I'll build a custom version and try it out. Edit: it works fine with this fix 👍