I think this can be more easily achieved by cating the .pid file in the data directory. Find the data directory:

2017-06-27 14:13:56.686000 TestFramework (INFO): Initializing test directory /tmp/user/1000/testo9vsdjo3

Then cat the .pid file:

cat /tmp/user/1000/testo9vsdjo3/node0/regtest/bitcoind.pid
gdb /home/example/bitcoind !$

(or if you really have to show off):

gdb /home/example/bitcoind $(cat /tmp/user/1000/testo9vsdjo3/node0/regtest/bitcoind.pid)

perhaps also mention that this may require superuser privileges, so may need sudo gdb ...

I assume most will get "permission denied" and sudo but maybe it's best to say

Yeah - if gdb can't attach to a specific PID, the error message is actually buried in the middle of the gdb text so it's not immediately obvious what the problem is. I think including the warning is helpful here.

Also, my above invocation was wrong:

cat /tmp/user/1000/testo9vsdjo3/node0/regtest/bitcoind.pid
gdb /home/example/bitcoind !$

it should be:

cat /tmp/user/1000/testo9vsdjo3/node0/regtest/bitcoind.pid
gdb /home/example/bitcoind <pid>

(sorry - trying to be too clever!)

I usually prefer

echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope

(on development machines)
to running gdb itself as root.