attach to running process without break:

gdb attach -ex cont -iex "set pagination off" $(pidof process_name)

set disassembly-flavor intel

define hook-stop
x/li $eip
x/8wx $esp
end

show bytes pointed by register with offset:

x/16bx $rsi+0x42

print array with size as bytes in registers:

p/x *(char *)$rcx@$dx
p/x (char[size]) *$rcx

break on address:

b *0xdeadbeef

break at offset to function name:

b *(&func_name+0xbeef)
b *(funcname+0xbeef)
b *('class::method'+0xbeef)

set conditional breakpoint:

break dlopen if strcmp(file, "libc.so") == 0  

set logpoint:

break func_name
commands
  silent
  printf "msg: %d\n", *($rdi+0xbeef)
  cont
end