What exactly do you mean by gargabe Collection of requered method?
If you wan't to know if a certain method was interrupted by Garbage Collection, you can check that in the Breakdown Column of the PurePath Tree.
garbage collection in general is happening completely independent from method executions.
however, it is possible that garbage collections influence the execution time of methods, of course. especially when so-called stop-the-world GCs happen, basically the whole JVM/CLR is halted and as such all currently executed methods are affected by this.
this is also exactly what AppMon is showing as "suspension time", e.g. in the breakdown column in the PurePath dashlet, as mentioned by Patrick above.
Additionally to the answer of Christian and Patrick: Have a look at the suspension dashlet (you can find it under diagnose events) to see all methods (with a sensor) influenced by a stop-the-world garbage collection and what the impact was on the specific method. This might also help you with figuring out which methods are generating the most garbage, as they are more likely to trigger a garbage collection.