I need to make load testing on my persistent storages Postgres and MongoDB.
I'm wondering if it is possible to do with gatling? Because as I see gating supports only http and jms out of the box.
Unfortunately no, it was under discussion some time ago, however the functionality didn't appear to have high enough demand from the community hence it was discarded.
You can use Apache JMeter for database testing, it comes with MongoDB Script sampler for Mongo and JDBC Request sampler for any other databases which support JDBC protocol.
New JMeter tests and existing Gatling tests can be combined into a single test harness via i.e. Taurus tool which supports JMeter, Gatling and few more underlying load testing tools.