In MN as in most states, you need a class A CDL when pulling a gooseneck trailer behind a pickup.

Not true. If the pickup GWVR and the gooseneck trailer GVWR is under 26,000 you can operate with a MN Class D license (171.02 Sub 2 7.ii.)

171.02 LICENSES; TYPES, ENDORSEMENTS, RESTRICTIONS.

Subd. 2.Driver's license classifications, endorsements, exemptions.

 (a) Drivers' licenses are classified according to the types of vehicles that may be driven by the holder of each type or class of license. The commissioner may, as appropriate, subdivide the classes listed in this subdivision and issue licenses classified accordingly.

(b) Except as provided in this subdivision and subdivision 2a, no class of license is valid to operate a motorcycle, school bus, tank vehicle, double-trailer or triple-trailer combination, vehicle transporting hazardous materials, or bus, unless so endorsed. There are four general classes of licenses as described in paragraphs (c) through (f).

(c) Class D drivers' licenses are valid for:

(1) operating all farm trucks if the farm truck is:

(i) controlled and operated by a farmer, including operation by an immediate family member or an employee of the farmer;

(ii) used to transport agricultural products, farm machinery, or farm supplies, including hazardous materials, to or from a farm;

(iii) not used in the operations of a common or contract motor carrier as governed by Code of Federal Regulations, title 49, part 365; and

(iv) used within 150 miles of the farm;

(2) notwithstanding paragraph (b), operating an authorized emergency vehicle, as defined in section 169.011, subdivision 3, whether or not in excess of 26,000 pounds gross vehicle weight;

(3) operating a recreational vehicle as defined in section 168.002, subdivision 27, that is operated for personal use;

(4) operating all single-unit vehicles except vehicles with a gross vehicle weight of more than 26,000 pounds, vehicles designed to carry more than 15 passengers including the driver, and vehicles that carry hazardous materials;

(5) notwithstanding paragraph (d), operating a type A school bus or a multifunction school activity bus without a school bus endorsement if the requirements of subdivision 2a are satisfied, as determined by the commissioner;

(6) operating any vehicle or combination of vehicles when operated by a licensed peace officer while on duty;

(7) towing vehicles if:

(i) the towed vehicles have a gross vehicle weight of 10,000 pounds or less; or

(ii) the towed vehicles have a gross vehicle weight of more than 10,000 pounds and the combination of vehicles has a gross vehicle weight of 26,000 pounds or less;

(8) operating a covered farm vehicle as defined in Code of Federal Regulations, title 49, section 390.5, that is not carrying hazardous materials of a type or quantity that requires the vehicle to be placarded in accordance with Code of Federal Regulations, title 49, section 172.504; and

(9) operating an autocycle.

(d) Class C drivers' licenses are valid for:

(1) operating class D motor vehicles;

(2) with a hazardous materials endorsement, operating class D vehicles to transport hazardous materials;

(3) with a passenger endorsement, operating buses; and

(4) with a passenger endorsement and school bus endorsement, operating school buses.

(e) Class B drivers' licenses are valid for:

(1) operating all class C motor vehicles, class D motor vehicles, and all other single-unit motor vehicles including, with a passenger endorsement, buses; and

(2) towing only vehicles with a gross vehicle weight of 10,000 pounds or less.

(f) Class A drivers' licenses are valid for operating any vehicle or combination of vehicles.

BTW if you are a farmer operating a CFV (171.02 Sub 2 8) you only need a Class D.

Based on 171.02 Sub 2 7.i. you can legally operate up to to 36,000lb with a class D (26,000 GWVR power unit with a 10,000 GVWR trailer)