Couple of questions:
Did your GM say they would get you in or that they would try to get you in?
Is your GM the raid leader?
As a raid leader I would never tell someone I will get them in, I will often say I will try but there are a huge number of reasons not to let someone in. For example if at the appropriate time for them to come in they aren't on vent with their character sat outside the instance i'm not going to break the flow of the instance waiting for them to get their ass over. Also if a replacement shows great skill, focus and attentiveness they may get rewarded by being kept in for the boss they really need. Its also possible that most of the loot you want would be better off going to someone else and it was just bad luck that the one piece you wanted and would be considered for actually dropped, your replacement might have wanted 3 pieces of loot 2 of which would get disenchanted.
As for your GM not replying to a whisper, its very easy especially if they appear during a boss fight to miss whispers. If you said "I'm ready for whenever you need me" that implies that you are happy to sit out if they don't need you, and they obviously didn't need you.
In all honesty if you are questioning leaving the guild due to not getting a reply from one whisper and not being taken in for one boss then as a raid leader I'd start looking for a replacement for you. Raiding teams work because they are a team not because one person feels entitled to a spot. If your schedule is going to make you unreliable then they are going to want to gear up someone on the bench if they are likely to stand in for you. If you feel you are no longer needed then you are hopefully correct, in a good raid team no one person is needed. If you are going to get upset about being sat for a 1 farm boss then you are going to have problems and its probably better that you leave now.