I'm used to debugging through comments, delete(to find which code has segmentation fault or cause Endless loop), and output(to find if the value is correct). And I think it is enough for algorithm competitions. Breakpoint debugging is too slow. But if you want to learn more debugging skills. You could watch this video from our school.

ahahhaah firstly, i read title like "debug my gcd or die".

I personally use multiple compiler flags which help me cover different runtime errors. It helps in detecting overflows, vector out of bounds ,stack overflow ,divide by zero by giving a better description of issue rather than just segmentation fault.

Compiler flags: -Wall -Wextra -pedantic -std=c++11 -O2 -Wshadow -Wformat=2 -Wfloat-equal -Wconversion -Wlogical-op -Wshift-overflow=2 -Wduplicated-cond -Wcast-qual -Wcast-align -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC -D_FORTIFY_SOURCE=2 -fsanitize=address -fsanitize=undefined -fno-sanitize-recover -fstack-protector

Note: Using these compiler flags will make the program run slower during testing.

For more information on debugging : Compilation and Debugging

is any offline version for it

Great website for debugging visually