In January 1989, he began his basic military service in the National Air Defense Forces. In the fall of 1991, he began his service in the GROM Military Unit; he was in the Reconnaissance, Operations and Combat Teams cells. In 1993 he was part of a team tasked with creating a Combat Boat Group in GROM. In 1994, Gąstal took part in Operation Restore Democracy in Haiti. In 1996 he was deployed as part of the United Nations UNTAES mission in Slavonia. Participated in the 1997 operation to capture Slavko Dokmanovic (Operation Little Flower). In the following years, he took a number of additional courses and training in the United States and Germany. In 2008, at the Naval Postgraduate School in Monterey, he received a master’s degree in National Security Studies – Counterterrorism Policy and Strategy.
In 1999 he took part in the OSCE KVM mission in Kosovo, during which he performed personal protection duties for Mission Chief William Walker. In 2003 Col. Gąstal participated in Operation Iraqi Freedom as an operations officer, working with Navy Seals and Army Special Forces. Upon his return from Iraq in 2004, he was entrusted with the creation of the Joint Task Force for Airborne Fire Support (JTAC) Guidance Group. In August 2011, the Minister of Defense appointed Gąstal as commander of the JW GROM. He held the position until September 8, 2016