You don't need to know anything about FreeBasic, unless you decide to re-compile the GCBasic source code.
A servo needs to only update every 20ms, so long as that happens, looping inline code should be fine. If delay routines, synchronous comms, state machines, or timing critical operations are required, then interrupts could/should be utilized. Also, many operations can run in the background like comparators, pwm, timers etc. so that little impact is noticed in code timing.