As you have gone to trouble of posting about this, I choose to give OP benefit of the doubt that receptionist was being sarcastic.
Possible she interpreted your bye and way as a sort of ‘thankyou’ and that’s why she said ‘you’re welcome’.
If she was being snarky then it’s possible she had her period, felt inferior as you look really good, feels inferior because you walked in wearing nice work clothing, thinks you seem aloof based on her own insecurities, saw a man check you out and feels jealous, or just has some weird belief that as the gatekeeper she deserves thanks.
In any case it all stems from her own weirdness so there’s no need to make yourself a weirdo too by getting down and dirty and absorbing this. You can’t go through life absorbing every exchange because if you’re sensitive you’re going to notice 10,000 times more weird things than the average person and that’s just energy zapping.
You need to step back and realise people are generally weird, misunderstood, flawed communicators, inadequate and have all their own stuff.
Get on with your life. You’re at the gym doing great things for yourself, stick to your lane and goals.