Build status:
For Debian Stretch see contrib/stretch.deps.sh
In general you will need node, yarn, python, make, swig and the build tools required by https://github.com/ElementsProject/libwally-core
See .travis.yml and .gitlab-ci.yml for the most up to date build configuration. In general, once you have satisfied the dependencies you should only need to:
For TESTNET run ./prepare.sh testnet
For MAINNET run ./prepare.sh mainnet
For TESTNET DEVELOPMENT run ./prepare.sh testnet development
For MAINNET DEVELOPMENT run ./prepare.sh mainnet development
Before making a pull request for WalletElectron check if what you want to modify is present in https://github.com/greenaddress/GreenAddressWebFiles - if it is then you should do the PR there.