Please excuse me I don't know much about this.

The problem is when I shift gears I get some resistance. It's like if it blocked for a second before going into gear.
If I'm stopped it shifts fine. It's only when I'm driving that it happens. It still does shift and seems to drive fine, just when I shift it feels like something is blocking it.

I drive a 2001 GMC Sierra (6.6 Turbo Diesel Duramax engine). It has about 120000 km on it.

most cars have something that keeps you from shifting at high RPMs??? Nice truck btw. Did it just start doing this or is it something that has happened since you've owned it???

It just started doing that recently. I don't think its normal.

The blocking rings are worn inside the transmission. The fluid should be checked right away and if the problem starts to severely effect driveability I'd consider having it rebuilt. Just putting new synchronizers in it won't be horribly expensive as opposed to what a normal overhual with new bearings and such would be, but it still won't be cheap.

There are a number of GM service bullitens about this trans. This one looks the most relevant:
Recently, a study was completed to determine the causes of slipping/missing 2nd, 3rd or 4th gear in the 4L60/4L65-E transmission family. This study was conducted using transmissions which were determined by dealership technicians as having a slipping/missing shift. The transmissions used were from vehicles with less than 19,000 km (12,000 mi) and the slipping/missing shift condition was verified during a test drive by dealership personnel before the transmission was removed from the vehicle.

Eight items were found to cause the majority of the slipping/missing shift concerns. These eight items are listed below in the order of frequency that they were found.
• Chips in the servo feed oil passage, orifice #7 in the spacer plate.
• Cut forward clutch piston inner seal.
• Cut 3/4 clutch piston outer seal.
• Input housing orifice cup plug blown out.
• Input housing cracked or broken.
• Mislocated valve body to spacer plate check ball(s).
• Case damaged at the 2nd clutch apply piston pin bore causing the pin to drag/stick in the bore.
• Input housing air bleed orifice cup plug with a check ball that does not seal.

In addition to the above items, a misadjusted shift cable has been found to cause these types of concerns.

When attempting to correct a slipping/missing second, third or fourth gear, the items listed above should be inspected as possible causes. It is also important to reference the appropriate Service Information for additional possible causes of these concerns.

first off....Someone should have asked: Do you have the Allison 5speed or the 4L80E?

Please excuse me I don't know much about this.

The problem is when I shift gears I get some resistance. It's like if it blocked for a second before going into gear.
If I'm stopped it shifts fine. It's only when I'm driving that it happens. It still does shift and seems to drive fine, just when I shift it feels like something is blocking it.

I drive a 2001 GMC Sierra (6.6 Turbo Diesel Duramax engine). It has about 120000 km on it.

OMG! My Jimmy has this exact same problem. Ever since I had my transmission fixed (I blew out the reverse by putting it into reverse and flooring it. yes i'm an idiot, sue me.) Every once and a while it will feel like theres resistance and feel like its in neutral. Id have it looked at again but its so god damn expensive to do anything with a transmission so untill it breaks i don't care. It only happens once or twice a month.