It has value as a shooter only - not a collector piece. $100-$200.
I'm not too sure of the value, but a Glenfield is a low end Marlin produced to make them more affordable, I own one and have become quite attached to it, the only complaint I have is the quality of the ammo clip. UPDATED ANSWER - A used model 20/25 in good or better condition (80%+) has a value of around $70-$120.
Only when you want to flirt with disaster: I would not recomend it.
.30-30 ONLY Other than that, I'd stick with good quality ammo...
To see the Used Value (Blue Book Value) of your gun you can go to www.UsedPrice.com All Glenfield firearms are really Marlins, so look for a manual for a Marlin 75. Value will not be much, I'd guess $100-$150 retail. Blue Book cross-references to Marlin 989M1, but the M1 isn't in the listings, but the basic 989 production started in 1962 and 989M2 in 1966, so it's in that range. Of about 25 .22 Marlins listed, only 4 have a price range other than $65-$135, and they're not far off. I had a model 75 marlin glenfield I bought at Sears in the early 70's. It is a 10 shot semi auto similar to the model 60 but does not hold as many rounds. I paid around $70 for rifle and 4x scope. I live in a saltwater environment and this was a very dependeble and suprisingly accurate rifle for the price. It is LR shells only.
Your Glenfield Model 30GT is basically a Marlin 336 with a straight stock. If you go to Marlin firearms website you can download the manual there. Your 30GT was only made in 1979 and 1980 then discontinued. The T stood for Texan the G either Glenfield or Grand depending on who you talk to. Based on the 21 in the serial number (oldguns.net) your 30GT and mine were made in 1979. They were not as poular due to the difference in the stock design and only 7,737 were ever made. About $250 is a close estimate on value. They are excellent shooter, just not collectible even though rare.
.22 short, long, or long rifle rimfire ammunition only.
The Higgins 101.1 was made by Savage as a Model 94. It's only value is as a shooter.
Regretfully, it has value only of a utilitarian nature.
Impossible to value with only the serial number.
Then it was manufactured before January 1, 1969, when serial numbers were not required on long arms manufactured in the US.
Impossible to value with only the serial number.